Why not? You can connect people with more people to talk to over time. You can give them more things to talk about. You can make it easier to share things with people on whatsapp. You can add streaks to incentivize sending messages even if someone would otherwise not send one.

why ? You ain't getting paid per message.

Because the more people who use your platform the better it is. Having a user that logs on 1 time per month and sends a message to himself isn't a very valuable to the platform compared to someone who comes on the platform every day and sends 20 messages between 5 people every day. Proper who when meeting someone new recommend to talk to them over the app instead of a competitor.