Why gun owners change polymer guide rods for steel ones ??,...What's the difference ?

A fool and his money are soon parted. Lots of people are ready to sell you crap that you don't need.

In theory, steel guide rods are more durable and stiffer... but that doesn't make much of a difference in the real world.

Sometimes, they are added for weight. If you have a super light gun (like the Ruger LCP) and replace the light polymer guide rod with a steel one (like the Ruger LCP Custom), it can take the edge out of the muzzle flip and bite of the recoil.

Steel is stronger than polymer and much more heat resistant. Some people will replace them to cover a potential problem area. It's really a non-issue unless your one of those people that shoot 100,000 rounds a year for a single gun for every gun like how many say you should practice since they figure you have literally unlimited money and time like they do.

The steel guide rod on my XD bent?....have no idea how/when/or why.
I called Springfield.....and they were happy to send me a new one......it was plastic......but I never had another issue.

because they heard on youtube that polymer rods can melt and cause your gun to malfunction and require new part to work again.

the problem is, i doubt normal people would normally intentionally attempt to melt a glock by shooting it, and find out that the gun will only melt if you have a steel guide rod since the plastic one melts a little after you get the barrel to red-hot.

for the most part, the difference between steel and polymer is that polymer melts, steel doesn't. you want steel if you're making youtube videos of melting guns by shooting them. for everyone else polymer is just fine.

No real reason too, doesn't matter what the guide rod is made of, the gun will function exactly the same.