MailChimp is bulk email / SparkPost is transactional email (like really short automatic emails, such as "a friend gifted you a game key!").
The MailChimp API would be used to gather info about the subscribed users and to add them to a list when they register (I currently have to use my own API to do this). You would also use the API to get subscriber info - are they subscribed to the newsletter (perhaps for bonuses if they are)? Did they confirm their email? It's a REALLY simple json api.
SparkPost is the same - you can send an email. For example, when you report a player, can have it send an email to an admin, or a support ticket email. Or if they make a purchase, can have it send an email to them with info.
I have an API that does all of this -- feel free to use mine as a template. There are PlayFab recipes in here too: