WhenIsGood works really well even if you don't have a user account - but there a number of advantages of creating one.
The main one is that you get to manage lots of different events and responses in your account dashboard. Here's a screen shot:
If you are not logged in to an account when you create an event, the system will supply you with a "results code". You'll need this after people have responded so that you can see the matching times. If you organise lots of events, this means you need to keep track of lots of different results codes.
With an account, this isn't needed. You just log in and access any results from your dashboard. Events with responses have a link alongside called something like "3 so far".