Custom fields are there to help you learn more about your contacts.

With contact management there are certain standard fields that are required to begin a relationship - things like Name, Address, Email, Phone etc. cant be left blank.

However, if you want to segment your data based on categories, products they're interested in, their birthday, the date they first signed up with you etc. then you should use custom fields accordingly. 

So which custom fields you need depends on how you treat or plan to treat your customer database.