Whilst I've been playing ArM for more years than I care to mention, I'm now running a new 5th ed game set in the Rhine and I'm stuck for inspiration for a reason that Oculus Septentrionalis would be willing to support a new covenant in the Rhine gorge - other than just "because we like you" which seems very weak. I had the magi of OS prevaricate and delay their decision until I could come up with something suitable but I'm drawing a blank!
The Rhine is one of the greatest (and most heavily taxed) commercial highways of the middle ages. Perhaps Oculus Septentrionalis wants a "commercial partnership" with a covenant in the Rhine Gorge that will help expand its considerable commercial enterprise in that direction. This includes making sure that all those pesky robber barons do not take an undue cut on their profits, and making sure that the Quaesitores do not get upset about "interference in the affairs of mortals".
-maybe they have some sort of a bigger political plan and want to secure some votes for the next Grand Tribunal.
-maybe they know about some thread (a magical moster, a curse) and are sure this convent will fail. They then hope to claim the convents resources.
As per Direwolf... when in doubt politics. IT may not even favors owed to the PCs... it could be some sort of strange scheme meant to punish the closest other covenant by a collection of the other covenants. Magi are spiteful little beasts. they may be expecting it to fail, or plan on reneging on their support etc etc.
Being a partner could help yup, or being the guys that keep those robber barons straight. Maybe they need someone running something for them. Or they kjust need something from Fengheld and support this covenant just to get on well with the covenant that really interests them. I can see the verditius in Occulus developing some river barges and needing to test them. Or them having problems with the Loreley and see this as an opportunity to get someone to keep her on check quite cheaply (for them).