A very short version: the use of grammar is that you could not have asked this question without using grammar!
You are making one big assumption, and as a result, you miss the main point:
Grammar was devised by people trying to create a language.
Well, in some cases, this has actually happened. But in general his is just a huge misunderstanding. Grammar is made by the people that speak the language.
Language is what people use to communicate with each other. Over time, language evolves, it changes. And over a time of thousands of years, nobody ever wrote a grammar book! Well, probably that also had to do with writing not having been invented yet.
But even after we started to write, most people never saw a grammar book.
Still, everybody knows grammar rules! Because grammar is not what someone writes down for you to learn, it is how people use the words in a language to make meaningful sentences!
So, what's the use of grammar? It makes it possible for people to use a language. Without grammar, I would expect you to understand the meaning of the following sentence:
and walk come have john dog home some the oranges after
I have used all the words from a normal English sentence, but I have willingly removed that pesky grammar that makes it difficult. So my words are in no particular order, I did not conjugate any verbs, and for good measure I removed all punctuation.
Now let's see what happens when I apply grammar to those words:
After walking the dog, John came home and had some oranges.
Now, I am not going to give you a list of all the grammar rules I used to form that sentence, but there are a lot of them! I can use them without even knowing they exist!. Native speakers learn the grammar rules of their mother-tongue when they learn how to speak. Nobody teaches a baby how to conjugate verbs, yet they learn by copying and understanding patterns. A grammar book is nothing else than a collection of those same rules that children learn by themselves when they are quite young.
What makes it seem difficult is that those rules are different between languages, so the rules for English may look very new and unfamiliar compared to the rules of your native language. That may seem unfair and complicated, but trust me, an English speaker learning your language has the same problem.