why the cooling system of a car in Houston need to add ethylene glycol ?

Apart from lowering the freezing point of your coolant (which you may not need in Tx) Ethylene Glycol also lubricates the water-pump seals, protects the metals in your cooling system from the corrosive effects of water, helps controls cavitation and raises the boiling point of the coolant (in conjunction with a working pressure cap). And it's usually dyed a cool colour.

In Houston where the temperatures in the summer time are very warm, the antifreeze in the cooling system raises the boiling temperature of the cooling system as well as corrosion inhibitors (especially in aluminum components). If you had just water in the system, as soon as the temperature reached 212 F like when you are waiting at a stoplight, the system would boil, creating too much pressure. You would end up blowing all the coolant from the system, or worse, a hose, heater core, or radiator. If you needed to add plain water in an emergency, that is ok as long as you replace it with the proper 50/50 mixture as soon as possible to prevent other issues.

Antifreeze and coolant are the same thing. You need a 50/50 mixture of antifreeze and water. Besides keeping the cooling system from freezing the antifreeze also has a lubricant that keeps the water pump seal from frying and a rust inhibitor that keeps the inside of the motor from rusting up. If you run straight water in your vehicle you can expect to ruin the water pump and the heater core and water radiator will get plugged up with rust. So because you are trying to save $15 on a gallon of antifreeze you will cause over $2,500 in damage to your ride. Not too smart heh?!

Antifreeze is also a corrosion inhibitor and actually has a higher boiling point than water.
While it is not often there has been freezing temperatures in Houston and the air flow through the radiator gives you a wind chill factor.
Finally you may drive to other places and having to remember to refill your coolant before you go is not going to happen.

Antifreeze is not just about lowering the freezing point of water but it also contains rust inhibitors and lubricants to keep the seals and waterpump working as they're designed to.

As well as acting as an antifreeze it also acts as a corrosion inhibitor.

If you use straight water as coolant it will work OK for a while, then galvanic corrosion starts to occur between the different parts of the engine. Alloy parts get eaten away and dissolved, which forms sediment and blocks the radiator and cooling channels. After a few years the engine starts overheating or leaking coolant, and is probably beyond repair at that point.

You did ask WHY right? Regardless of where,

You can buy premixed coolant that uses Hybrid Organic Additive Technology or HOAT.type. (You may have to ask around in a price driven market.) but it is no secret.
You can use coolants that are then mixed with distilled water by the user.

The metals used in components that the coolant comes in contact with
are affected by its alkalinity.
Dissimilar metals in liquid solution with convection can produce a voltage.
If the coolant is allowed to become unprotected chemically, and acidic ?
It will begin to reduce the more ignoble metals.
It would tend to build up on the more noble.

Leaks result, however minute, the level decreases, the ability to produce air bubbles increases, the heat transfer is less, the numerous other factors results in some form of corrosion, cavitation, leads to overheat.
It leads to heat exchangers with reduced heat transfer.

So, the right use stuff makes water wetter,improves heat transfer, reduces these issues, protects the system from damage, and misdiagnosis, thermostat removal, the need for service and repair so often.

Ensuring system grounds and the ground straps are intact and well placed whether we realize it or not, we stop the slow destruction.
When you look up what is specified for your system, use it, and follow your manufacturer bulletins and updates, you stand a chance of exceeding the expected useable life of every component.

If you change something to your own idea?
Then you be responsible, buy the failing parts
till it exceeds your ability to do so.
It is best when you can trade off a vehicle while your systems are in good repair and their best properly maintained condition.

It is not expensive unless you ignore it or deviate from the recommended
specified and updated info about normal maintenance.
Manufacturers can have products they agree or disagree with the use of.
You can find out pretty fast with a call to your dealer parts counter.
Each company wants you to buy their product.
Find the ones that serve you best chemically by asking questions.
You can test your system yourself if you read up on the research
and be just as accurate with a few tool purchases.

In ONE word: Protection.

this is anti freeze. it is additionally an anti boil agent, it is also a corrosion control agt and finally it is a lubricant for seals. temps and area little matter

Antifreeze, (ethylene glycol) contains rust and corrosion inhibitors so the radiator, cylinder heads and engine block will not rust or corrode on the inside water cooling passages.

A 50/50 mix of distilled water and pure 100% antifreeze with a 50 lb. radiator pressure cap lowers the freeze point to -34 below zero F and raises the boiling point to +162 F.

Hi as the temperature drops water freezes as water freezes it expands inside a cylinder block of an engine it cracks the cylinder block and cylinder head. so destroying the engine. so by adding ethylene to the water in the cooling system this reduces the temperature at which water freezes. so it is less likely to damage your engine.

It stops corrosion

IT PREVENTS OVER HEATING and the formation of RUST and other contamination that would LWER engine life! It also protects GASKETS and other expensive parts in the engine! THERE IS NO UNIVERSAL COOLANT either! use ONLYL the coolant from your DEALERSHIP unless it is a GM car! Then use AUDI VIOLET or BMW BLUE! They were SUED over the action of DEXCOOL and GM gasket and plastic choices! THEY SHOULD REFUND EVERYONES MONEY FOR DEXCOOL DILEMMA! They do NOT care about you AFTER the sale BLAH BLAH BLAH!

Uh, that is the integral ingredient for antifreeze/coolant.