When does a web developer use cookies and session?

Question

using session and cookies

Answer 1

when logins are involved on your website.

lets say someone logins on your website, then you'll create a new session. but once the user exits your website. you remove the session for the user.

Answer 2

The main purpose of a cookie is to identify users and possibly prepare customized Web pages or to save site login information for you. When you enter a website using cookies, you may be asked to fill out a form providing personal information; like your name, email address, and interests.

Note:
This was totally not copyed and pasted from google:
https://www.webopedia.com/amp/DidYouKnow/Internet/all_about_cookies.asp