new to this - what is main purpose of using keyword in seo?

The keyword is just the words people type when they want what your page offers. Using it tells Google (and the reader skimming the headline) what the page is about, so it can be matched to that search. In practice: one main keyword per page, put it in the title, the H1 and early in the intro, then write naturally. Repeating it 20 times doesn't help and reads badly. The bigger job is picking the keyword before you write, so you're making the page people are actually searching for.