why should we use forms

Answer 1

Forms are used to provide a way to users to send information to the server, for a forum registration, posting a commentary, etc.

Answer 2

static pages were good back in the days, but thing evolved and clients interactions too. Imagine you run a web shop, and you want to make customers accounts, or what if you run a page and want to be contacted directly from that page, in those two cases you need to collect users info, and you do that by forms