What is fertilizer and how can it help?

Fertilizer is nutrients. If your soil if depleted and poor, it can help the plant be healither, because it has the nutrition they need for growing strong and healthy.

Sort of like the different between a person eating only crap, and a person eating a healthy, well-rounded diet with lots of whole foods.

If you have good rich soil, you may not need any. Though if you plant the same plants over and over in the same spot, they will take the nutrition they need from the soil, and if you don't replace with fertilizer of some kind (I use compost myself, not the commercial nitrogen-heavy fertilizers), they will get less and less of what they need over time, and not produce as well in future years.

Long answer. During WWII there were many explosives plants supplying bombs and the like. After the war there was NO NEED for explosives. What to do? Explosives use a lot of Ammonia. Plants need Nitrogen. So the explosives plants became fertilizer plants and they advertised heavily. You need fertilizer by ton. No you don't. I fertilize my lawn every 5 years or so and it does great. I use a tbsp of 10-10-10 per plant in my garden twice a year per plant and all is fine. Fertilizer is overrated and is actually not good in abundance. If you use too much, plants get weak like you would if you sat in a chair and overate. Plants did well for millions of years without humans fertilizing, what a miracle.