Question

why do we use hyphen for some words

Answer 1

Because They notify the reader that two or more elements in a sentence are linked and if hyphen is not used then it may confuse the reader that what actually is the writer trying to say. For example --

I am living in Los Angeles for 8-10 years

and if without the hyphen ---

a.) I am living in Los Angeles for 810 years ( that is simply not possible )

and hyphen is also used sometimes to shorten the length of your sentence so that is is short and simple and easy to understand