Ideally you’d never need to send one as we’d be totally on top of everything 
If you don’t hear back from us in the timeframe you’d expect to, a follow-up or chaser is helpful - occasionally (rarely) emails triggered from our hiring software don’t arrive, in which case a follow-up from you is the only way we can know this and make sure that you actually hear back from us.
We track where candidates are in the process and how long they’ve been in that stage for, and are working to make the whole process quicker, but it’s helpful to get human signal when things aren’t working as expected - we love feedback.