"Compelled by an outside force" is the main fear for many people. Because it happens all the time, and some of those instances also have an NSL / gag order so they're unable to talk about it until years after the fact (if ever). Or they just threaten violence.

Threat models aren't the same person-to-person - this probably won't happen to you (the grandparent), but embedded journalists / people trying to overthrow a corrupt regime depend on this stuff to literally keep them alive.

Another fairly common possibility, and one that affects damn near everybody: they can get hacked and have their source code modified. This happens with some regularity, and it can affect apps too: https://www.macrumors.com/2017/05/07/handbrake-app-security-... but in a browser this happens silently and unpreventably. Apps don't (usually) update invisibly just because you launched them.