[ahelwer]

A bit taken aback this still needs to be explained, but the point of having an account is so they can use your email address and phone number to correlate you & your actions across the entire internet to better surveil you & serve you behavior-modifying content.

[goodpoint]

The "free" service is also there to hurt competition, establish market dominance and later or do all sort of bait-and-switch.

Reminder: by dominating a market a company removes options and better alternatives from the end users.