If by "multi-recipient emails" you talk about newsletter that's probably more MailChimp's territory which is also clear from their landing page. Mailgun is clearly aimed at developers implementing transactional email functionality.
It's not "necessary", same as Stripe isn't necessary for accepting payments but it makes it a whole lot easier if you have to deal with it and don't want to implement everything yourself or run mail infrastructure.
It's not "necessary", same as Stripe isn't necessary for accepting payments but it makes it a whole lot easier if you have to deal with it and don't want to implement everything yourself or run mail infrastructure.