Why add an account?
Adding your online accounts brings your choice of services like calendar, chat, and e-mail straight to your desktop and makes the information of the services a seamless part of your user experience. By adding accounts you can easily keep in touch using services from different accounts, like chats, at the same time. Just set up your online accounts once and every time you start your computer all the accounts and services that you have added are ready to use.
Seefor information on which applications can access which online services.