they are no more based on xmpp and diverged from long time. But, basically, the reason I use them is network effect. Othere people use it, so I use it to communicate with them.

Frankly I hate signal cause is a "just slightly better wallet garden" but noone uses it. So is another shitty app on my phone to talk to two guys, I hate it more than whatsapp that at least permit me to do backdoored communications with all my contacts.