A quiet way to stop unwanted contact
If messages from someone have become unwelcome, blocking stops new messages from reaching you without sending them a direct notification that they've been blocked.
Blocking is stronger than just deleting a contact
Removing someone from your contact list only takes them out of that list -- they can generally still message you afterward. Blocking is a stronger step that actually prevents new messages from getting through at all.
Frequently Asked Questions
Can the other person tell they have been blocked?
There is usually no direct notification, though changes like their messages no longer showing a delivery confirmation, or an outdated profile photo, can sometimes tip them off indirectly.
What is the difference between blocking and removing a contact?
Removing a contact just takes them off your list, but they can still message you; blocking actively prevents their messages from reaching you at all.