Each account you add is like a mini portfolio. A portfolio holds each of these accounts with the idea that you use a single portfolio for yourself and if you need to separate any of your stocks you can use a different accounts.
Accounts can be used for several different things. The main one being to separate your long term and short term trading but you could also use it if you run different brokerage accounts, margin loans or have stocks under another name. They can also be used to group stocks of the same currency together when trading on multiple markets.