Grammar talks about language, it explains how language works and has invented a series of useful grammar terms in order to be able to talk about language.
If you don't know anything about grammar, you can talk English as a native or a not-native speaker, but you can't check whether your English is correct or not.
With a grammar in bookform or online you get a lot of information about correct and not correct English.
Of course, you have to learn the terms grammars use, and that is no easy task, but it is worthwhile. Using a grammar is no easy task either. My experience is when schools don't teach how to handle a grammar a beginner is helpless. Those who use grammars are mostly those who have learnt Latin or another foreign language.
If you don't know the grammar terms it is like knowing the ten digits without knowing the mathematical operations like plus, minus, multiplied with, divided by etc.
My recommendation for you would be try to learn how to use a grammar, to find something in such a grammar by using the table of contents and the register/index. Grammar terms should be explained in a grammar and be listed in the register, so that you can find the definition of the terms like word classes, the parts of a sentence, word groups, and a lot more. Unfortunately, I must say explanations of grammar terms in English grammars are often of very low quality, often too short, and using a lot of confusing terms or the explanations are simply not optimal. English grammars can't compare with a Latin grammar.
But you don't study a grammar in detail and then begin reading. You start reading and when you find things where you are unsure then you try to find that point in a grammar and try to understand it. If need be you can ask here and get information about your problem and where to find it in a grammar. That is sometimes a bit difficult because grammar things often have two or even three names. You should only use the simplest and clearest names and no fantasy terms.
And if you get a grammar don't buy such books with a lot of exercises. Real grammars don't have exercises in them. They explain a grammar point, such as to-infinitive or gerund after a verb, give examples and when necessary lists.