[vel0city]

Using it over WiFi is big feature alone for a lot of people who are often in spaces with good WiFi but poor cellular reception. Some carriers support SMS over WiFi in the same way they do "WiFi calling" which can alleviate that.

The other major feature I see for other messengers over SMS/MMS is much larger attachment sizes. It can be challenging sending an MMS with attachments >1MB. Meanwhile, I can send a 100MB file/video or an 8MB photo over Signal. WhatsApp allows for 16MB attachments. Sending a quick video in-line with the chat thread in MMS is miserable and gives an incredibly trash quality video while most other chat apps you can stick a decent quality 30 seconds or so video without any issue. Photos sent over MMS are usually junk while one could get a decent 4x6+ print off an 8MB photo.