1. Coverage means having somewhere to go
It's one thing to have a bus line somewhere near you. It's another to have one that goes where you need (at different levels, of course: everywhere you need, most places you need, at least somewhere you need). There are extreme examples; I've visited towns that have only one bus that takes you down main street (or equivalent) and to the town over.
This is great as a start; it means you can get to the town over! But if you were going somewhere else in town, or to a different town over in a different direction, it's not helpful. It's purely binary; the destination is or isn't on that one bus line.
Even in places with more than one bus line, I've encountered some major gaps; one time I couldn't get to the mall, for some reason and another time I had to walk a mile without sidewalks (a different but related kind of urban accessibility issue). If you can't reasonably get where you need, the existence of a bus isn't actually helping you.
2. Coverage means being able to rely on it
I don't necessarily mean you need to be able to never own a car (though that's nice when possible!). I mean that if you take the bus there it needs to get you back that time, since you took the bus and not the car. This means hours of service (if the bus stops running at 5 and I miss that bus, I might be out of luck for over 12 days). It means frequency of service (if I have to wait another hour if I miss a bus by a minute, that reduces my willingness to rely on it). It means visibility (do I know when and where the bus is supposed to come? Or whether it's delayed or rerouted or something else?). All of these are necessary for functional coverage.
3. Coverage means effective trips
This is related to both of the other points--and, for all the other issues the Davenport Citibus has, I think it does this fairly well. Coverage means actually being able to use those lines on the map: connections between buses that are decently timed, located, and run; no surprising routes that don't actually run at a certain time; ideally, connections that aren't all "go downtown and then transfer."
Rochester, NY, for instance, has very few suburb to suburb service options, so even though one can theoretically go from neighboring suburbs to each other, it may be an hour in to the center and an hour out for what is a fifteen minute drive. Davenport itself does better than this, with other transfer points around town, and the same is true of the Metro on the other side of the river, but of course in the QC as a whole intersections between Iowa and Illinois suffer. As always.
These are just some thoughts on why just seeing lines on a map doesn't actually mean you can (or people will) use a service. Good transit means more than a good fold-out map in a brochure or on a bus stop wall. It means a service where that coverage has meaning and use for people, not just maps.