This is something I push all of my clients to do, for the below reasons at least.
It’s your own audience
When you have the name and the email address of an individual, then it’s your audience, not Zuckerberg’s or Daniel Ek’s, but yours.
You know who your fans are
When you do a mailshot using email marketing software (e.g. Mailchimp), you know who clicked what and when.
Everyone has email
At least everyone on the internet anyway. Plus anyone on social media (usually) has to use an email address as they usually need one to make an account.
As before, you know your fans by name and by behaviour. You can drill down in your campaign reports to find out exactly which individuals opens and clicked links inside of your emails.
You can format your emails however you like, and sell things however you like! Every email can have call-to-actions (links) to your music and your merch products. You can embed photos and videos with ease.
When you build an audience, you build leverage. If/when it does come to signing a label deal, you’ll have more to bring to the table and thus have a better negotiating position. This is leverage.
When someone goes from a mere follower on social media to an email subscriber, they have given you permission to communicate more directly with them. This is not to be trivialised.
For all of the above reasons, as well as being a more direct form of marketing, email is a far more effective means of selling and promoting things. Soon you might not even bother posting on socials, not that I am recommending that!