Hospital units give you the ability to recover hits faster, I believe it's 10%, maybe 15%. This is only to the formation that it's in.
Signal units give 2 more command points to the particular stack they are in.
So, in other words, they can help any size formation from brigades all the way to an Army. Signal units can help cut down the command penalty when a formation goes beyond the maximum ability of the highest ranking officer which, I believe, can include an aide-de-camp. This would be at least a 2 star general. I don't think 1 star generals can help in formations larger than a division.
Sometimes I'll form a division with a 1 star general & then put a higher ranking 1 star generals (with better abilities) in charge & this eliminates the 10% command penalty for a division. Corps are a little different; of course the general's ratings ar4e important but they're affected by the general who commands the army that the corps is attached to. It's best to have a strategic rating of four or more to really help in the aide-de-camp position.
Sometimes you will find it best to move these two units (Hospital & signal) to different formations because you may need them in another formation. What I do is use the hospitals in the rear & be available for whatever units have taken hits & I want them to recover hits from battles. It's best to keep the signal unit with the formations that are overtaxing their command points.
I hope that's clear enough for you.
Oh yeah, forgot to add that these are not cumulative; in other words, there's no benefit from putting two hospital units or two signal units within the same formation; only one is beneficial.