During Momentum's account creation process, after entering your email, you will be asked to choose a password. The password you choose will be used along with your email address to login to your Momentum account.
A Momentum account automatically backs up your Momentum data, so you won't lose any of its stored Momentum data if your computer runs into any problems. Additionally, it allows you to login from other computers and see all of your stored Momentum data (Focus, Todos, Links, etc.).