I balk at any mention of crypto as a rule (since the landscape is so saturated with hucksters) but I have to assume it is to provide a functionality similar to WhatsApp Pay, Venmo and whatever WeChat has.
Those in app type payments are a huge part of message app usage in some parts of the world where I am sure Signal would like to increase uptake.
Mobilecoin is a quasi-security and not a stablecoin. The price went 5X in the days before Signal announced that they have built MobileCoin integration (in secret).
In a way, crypto has given non-profits a way to take profits out to personal accounts.
Those in app type payments are a huge part of message app usage in some parts of the world where I am sure Signal would like to increase uptake.