In universe it's supposed to like strengthen cell membranes.
Since a lot of issues stem from cell membranes being pierced, damaged, etc. that should be fairly generally effective. Especially as a first step to try against an unknown.
Came here to say this. It's probably a drug that's dead useful in a lot of cases (there's a few of these today in emergency medicine), and so gets given a lot.
I think it is the opposite for something ready to be administered. Sure, a certain mass of the active ingredient is suspended in a specific volume of an inactive medium like saline. But when a physician is ready to administer it, it is much easier to portion out the desired volume of medicine than to weigh it out. As long as the mixture has an established ratio of active ingredient to inactive medium, measuring out the desired volume should be quick and effective.
It could be that the inaprovaline hypospray they use has a ratio of 1mg per 1cc, and that everyone knows that. Meaning that one could say either "20 mg" or "20 cc" and they would be asking for the exact same quantity of the active ingredient.
It turns out that removing targ worms drastically increases your odds of surviving most afflictions and ailments, so it’s a standard practice for the EMH mk I for every issue.
The EMH mk II discontinued this practice but most crew members find him off putting and avoid him.
IIRC, there are quite a few episodes of Voyager showing the hypospray being loaded/reloaded with a cartridge. It's unclear how many doses are contained in the cartridge, but typically more than one. I would think that for most treatments the Sickbay replicator just produces a ready-to-use batch of cartridges, whereas for more specialised medication the EMH has to actually make it up manually.
They can be configured for different medications on a single cartridge though, it seems like, so it must not be just medicine, but some kind of solution of components that the case can mix