Based on photos of poorly executed paste jobs, I'm guessing that people who aren't very dextrous will benefit from this tool. There are a few people on this site who might want to consider buying one of these.
This also might be handing for those who paste CPUs frequently. Freehanding the thermal paste takes a little more time.
Pastry chefs have similar templates for a variety of usages (tuiles, chocolate, etc.). While this can all be freehanded, if you have to do a lot, the time adds up. I've seen stencils in other crafts as well, this sort of thing has a long history.
Heck, I wouldn't be surprised if something like this is being used on the assembly line by OEMs and system builders to speed up the process.