Sendgrid is an email service that can be used with an API and nearly any site can use. This type of mail service is called a "transactional" email provider. Most mail going from a site will be related to the service that site provides. For a learning site for example, this could include course reminders, invoices and receipts, progress reports. Some "gray-area" items that could be marketing mail, like new course announcements, can also be sent through such a service.
Using a transactional email service provides more control over how your email is delivered to your audience. Stats of how many mails were sent, what was opened (if supported), and even delivery logs are available with their panels.
Some anti-spam filters can see the gray-area emails being spammish, separating your regular mail from the transactional email ensures your regular mail is still deliverable and that you can still communicate with your clients.
Most times you can also use your own regular email service to send mail out, however, for best security practices it is best to use a specific user (mailbox) for this specific purpose. Some mail providers (google, microsoft, zoho, etc) charge fees for every user that you have. Using these services for transactional email can be a risk, however, as they may see the mail as spam and terminate your services because of that. This goes back to the gray-area mail that is still business to client but some may see as spam. Also, web sites can generate a lot of mail on their own at times and some mail providers may have a sending limit that a transactional email provider would not.
