Because power
Also I don't think he fell in love with the mc yet, it's more like he's still the the phase of slowly developing feelings for her, as most of his moves are sort of calculative on how to seduce Latil.
That's why I said it's not because he loves her.
As for powers...I disagree. He's a concubine. Before that he was either first or second in line to one of the most powerful houses of an entire empire. Being just one of the consorts of a harem of an empress that hasn't even truly consolidated her power is a step down. It's certainly a good gamble if you have no other options but he doesn't strike me as either desperate or particularly greedy.
unless he works very very hard to be her favorite and in such a varied harem that....simply isn't happening.
Even the readers think he's boring lol
Honestly I don't get why ranamun likes mc. I get why the others like her the way they do but I don't get him. It's not like he fell crazily in love like guesta, Carlein and sonnaought. It's not like he's just naive like the pope and the prince. It's not even that he's mysterious and just having fun like tasir.
So why the fuck is he there and why the fuck is he doing all that? He has pride and cares about his appearance so why would he do that to himself