At the risk of oversimplifying... Consider the example of a broadcast FM station licensed to operate at 100 kW ERP.
The "range" of the station is technically infinite, and not exactly related to the TX power. Assuming a transparent ionosphere, the signal will radiate outwards toward Zeta Reticulae at the speed of light or anywhere else in the universe given the cosmic alignment at the moment of observation. At 5 kW ERP the signal will travel exactly as far as it would at the 100 kW level assuming the same antenna in both cases. But, what will be different is the field strength at any given point within the pattern. Considering the size of only the terrestrial audience, "range" is more a function of antenna height than power. Raise the antenna height and you raise the look angle toward the horizon along with the area of real estate that can "see" the antenna.
So why does an FM broadcaster usually want to run the most power they can?
A significant portion of their audience is trying to listen on a cheap radio with a crappy antenna in a bad location. More power won't cure multipath distortion in the stereo image or penetrate down to the basement break room in an office building, but for the rest of us, hearing the commercials in full quieting is considered to be a good thing by the station sales department... Nothing worse than trying to sell air time on the weak signal outfit that no one listens to because of the weak signal.
Same deal with a VHF / UHF mobile.
Five watts in the clear will work a bunch better than a hundred watts while parked in a basement parking garage. A hundred watts will work better than five watts from a good location way out in the boonies. If your travels tend to be local to the repeater the added power won't do much for you. The RX will still squelch out under the railroad bridge on sixth street. If you're more the interstate highway type you will be able to hit repeaters further out than you could before, but will still drop out in the valleys between the hills.
The only thing the increased power will do fer'shure is answer the question: Could I have held the repeater from here if I had more power? Sometimes the answer will be yes, sometimes no.
BTW: In my travels I've found that a ten watt mobile works about as well as fifty watts since the RX tends to be the limiting factor. When a repeater starts to chop in and out with every change in the terrain it's time to go surfing for another repeater...