Normally if the local mail works for you you should not bother. The From: address for outgoing mail in this case will be something like email@example.com .
First reason to use SMTP is if you want to use the different From: address, like your google mail for example. In this case using local mail sever will result in your mails marked as spam by most spam-filters (they compare the From header and the real server that sent the mail)
The second reason is if you have lots of users and need to send lots of mails (say, more than a hundred per day). In this case, your server may be marked as spam source and besides it will be unnesessarily loaded. Using sendgrid or similar external mail server is a solution.