What different between using ASP.NET identity and custom log

Question

I m beginner in asp.net mvc

I need to make web application by using asp.net mvc5 code first to pharmaceutical company

have 1000 user . when starting to implement web application by mvc i starting by doing login by user id and password

but i search more in internet i found about login in mvc 5 asp.net identity

I have database and i have already table users have userid,password,role

why use asp.net identity and what different between asp.net identity mvc and custom login mvc

please help me