If you buy the domain through a company like Gandi you also get some free email storage and webmail as well. It isn't great but for the cost of a domain name you can get 5 mailboxes and it is quite a good deal. When you outgrow it host it with whoever you want and transition your mail storage.

Having your own domain gives you a lot of options to move your email around to the best deal and hosting at the time, companies are going to come and go and if you utilise free email a domain in front of it will avoid having to change 500 accounts on the internet when you inevitably have to move.

What happens when your registrar shuts you off?