I was wondering what the 'loths use the jink they receive from clients for - to buy weapons? Build fortresses? Or what?
That's a decent question.
Personally, since I consider the yugoloths an extension of the themes of the Grey Wastes; I always considered that they horded wealth the same way that Hades/Pluto does. Not to fulfill any specific need (although I could see a lot of their secret projects being funded this way) but rather to fight off a feeling that they are weak or emotionally empty without acquiring MORE (whether more wealth, more power or more souls)
Rather than deal with the limitations of their philosophies, they feel that if they can just gather "enough" then they will feel satisfied; never realizing that there is never "enough"
While I like this explanation, it doesn't open up the same amount of adventure ideas as some big project that they might be working on. Unfortunately, I don't have any good ideas for this later possibility.
This idea won't appeal to everyone; but since the yugoloths are from the Gray Wastes and that is all about death, I use the idea that the yugoloths are secretly harvesting death throughout the multiverse. This explains their fickle behavior in the Blood War. They don't really want either side to win but they do want them to keep fighting as this results in more deaths.
Even though most "dead" fiends will come back in one form or another, I imagined that the 'loths are still able to syphon off a fraction of energy from each death to power themselves.
Now this leads to the similar question of "What do the 'loths intend to do with the power they are secretly accumulating?" Again, I don't have any good answer but I like that this plan is sneaky and that it could justify a BIG power move by the 'loths if I could think of something worthy (e.g. enslaving both devils and demons? and/or attacking the Upper Planes?)
I suppose this is the unimaginative answer, but I would guess they spend it like any other cutter, on stuff: magic items for personal defense, materials for researching forbidden lore and dark spells, bribes for easily swayed pawns, and anything else that looks promising in terms of increasing their ability to gain future wealth and greater power.
Also remember that every yugoloth with any personal wealth has a small collection of items such as embarrassingly cute little guardinal action figures, saucy semi-animated flipbooks starring Erin Montgomery and Rhys, or autographed celebrity portraits of extra-famous bards. This costs a fair fraction of their wealth, and if ever revealed to another yugoloth they are forced into a servile relationship to prevent their secret from being revealed, despite the fact that they all have such a secret.
(I am joking. But it'd be funny.)