Two things every business sending email eventually needs: a way for customers to choose what they receive, and documents that arrive as PDFs.
Customers Choose What They Get
An unsubscribe link opens a preference page where customers turn individual email types on or off, rather than losing them from every list at once.
Invoices and Receipts as PDFs
Invoices and receipts are rendered as PDF documents, which is what customers expect to attach to a claim or forward to their own accounts team.
Your Own Sending Domain
Email sending is set up against your domain, which is what keeps your messages out of the spam folder.