I was admittedly confused about this distinction at one point too. It's a trade-off (although few people effected by this own phones with truly free, user-respecting soft/hardware in the first place).
Not really. Samsung was the first with this, but their reasoning had absolutely nothing to do with security. It was because their phones slowed down over time and their solution was to give users the option to reboot it at specific intervals. You could even make the argument that the Samsung solution is still the superior solution because you get to set the interval.
As the GrapheneOS docs note, the feature is better implemented in init and not in system server or the app/services layer like Google has done here? Though, I am sure Google engs know a thing or two about working around limitations that GrapheneOS developers may have hit (in keeping the timer going even after a soft reboot, where it is just the system server, and the rest of the userspace that depends on it, that's restarted).