It's all down to personal choice really. The majority of drivers who use dash cams only use a front facing cam because if you're rear-ended you'll rarely need to prove it wasn't your fault. Also, dual dashcams/two separate dash cams are expensive.

However, a rear dash cam could provide useful info in the event of a hit and run, like reg plate details or to evidence that the other driver demonstrated reckless driving just before hitting your car. It could also prove someone was on their phone at the wheel before they hit you. If you have to stop suddenly and get hit from behind, it could prove that someone was too close/tailgating you beforehand. Likewise, you may find people back off a bit when tailgating if they see a camera pointed at them.

If you can afford it, there's no reason not to have a front and rear dash cam, but make sure you read/watch plenty of reviews before buying. It's not uncommon for manufacturers to focus more on the front camera, so the rear camera often films in lower resolution and can be a bit rubbish.