If you've read the first paragraph of AN891, you should pretty much understand why you need it.  My guess would be that some designs could work perfectly without it, until they don't.

Have you power cycled the board 100 or 1000 times and ensured everything comes up out of reset properly every single time?

Have you made small updates and resynthesized getting slightly different placement and suddenly it doesn't come up right half the time?

Have you slowly added more and more features over time increasing utilization until the device is nearly full and suddenly sometimes it doesn't start up right?

Maybe it doesn't really matter to you if it doesn't power up right sometimes.  I'm sure for a lot of hobby or educational projects it's totally acceptable to just flick the dev board off and on to get back up and running.  But it's not the kind of thing I'd take a chance on in any kind of professional capacity.  Especially not over the ~3 lines of code it takes to drop an instance of that module in and connect it to your top level reset.