converting comment to answer:
I figure because they are more likely to win against non-GMs as opposed to other GMs. so it's a win-win: the non-GMs have a chance to prove themselves to get their norms, and the GMs have a good chance to win.
Grandmasters are financially incentivized to play in GM-norm invitational tournaments. The incentive can take different forms depending on the organisers' choice.
Most often, GM are invited (travel, hotel and meals paid, with no fee asked for playing) and they compete for the money prizes in the tournament (usually the top 3 out of 10 players get money ...
After several searches I can only find one source in which Granda is quoted about Grau's books. It's this part:
"Yo tenía muchas limitaciones técnicas, así que me ganó con bastante facilidad, pero fue muy generoso porque le dijo a mi padre ‘Che, al pibe le falta Grau’. Mi padre, que no tenía mucho conocimiento de ajedrez en ese entonces, preguntó qué ...
"Needs citation!" (xkcd)
That Wiki entry should be either disputed or given a reliable source. I googled a bit:
Chicago Tribune article
Sounds like from an interview, but I'm still not 100% satisfied.
Another link, the danger is everyone is copypasting from everyone else. E.g. here would be the place I expect that it would be mentioned, but it isn'...
Update: Ah, see the list of all grandmasters. This has a death date column. Click on sex to get all the F's together and then see they're alive.
Wait wiki page for Zhansaya Abdumalik, the most recent female grandmaster and the most recent grandmaster, says (but it's unsourced)
She is the first Kazkakhstani woman, and the 38th woman overall, to earn the GM ...