Because you can send a message to a /person/ (or multiple people) instead of a /phone number/. Depending on said service, said person can interact with the message independent of device or phone number. Useful for people who interact with people who change phone numbers/devices often and/or travel between countries. Just send the message, and you don't have to think.

But people will forever stick to their stock messaging application as much as possible, sticking to slower, more restrictive technologies. MMS is a dinosaur.