Most people don't
need it, but there are reasons it can be convenient. And some just like to do everything in-house either as a learning exercise or for control freakery reasons.
Control of TTL values is on example. Most registrars use 4 hours these days but it used to be that 24 hours was the value used by most (with not option of anything else). That can be a minor convenience if you expect to move things around much. I have all mine set to 5 minutes (not a great idea for anything high traffic, but nothing of mine is). If you know what you are doing running a small DNS service is no great hardship at all (though it is surprising how many people don't get it right). Custom dDNS is another reason you might want this (though I think some registrars and specialist DNS hosts offer this for little or no cost these days).
In my case it costs nothing as the three bind instances I run live on geographically separate machines that I already have for other reasons (home line, external web service, backup location) - if you have no external resources already then you'd need to pay for somewhere to host a secondary server of course (cheap, reliable enough, and fast enough VPS services are common so that needn't be much cost - though a specialist DNS services needn't any more expensive these days either and will likely be more scalable than anything we setup manually).