One reason for PAR is that new lighting technologies on the market changed the game. Watts per gallon was fine as long as everyone more-or-less understood how much light came out of an actual watt. Halides and fluorescent variants (NO, HO, VHO, T5, PC, etc) all were more or less of similar efficiency, so even though watt - as we look at it - is only a measure of power draw, it could be more or less correlated to a unit of light output. It's just like old-school incandescent lamps... we don't say "Bring me a 750 lumen bulb," we say "Bring me a 100w bulb," even though lumens is a measure of light output, not watts - but everyone pretty much understands how much light comes out of a 100w lamp vs. a 60w, etc.
Nowadays though, some lighting technologies (most namely LED), have rewritten the book, because if you put X watts of LED next to X watts of halide or fluorescent, the difference would be enormous because LEDs are so much more efficient at converting those watts into light output.
The industry has turned to reflector technology as a big differentiating factor in the performance of a lighting system. A system with a crap reflector but great lamps won't perform as well as one with great reflectors and no-so-great lamps. PAR can take these differences into account.
Also, PAR is a standardized unit of measure, which can be used to compare different makes/models of lamps. For example, watts per gallon might be fine to guestimate how many watts of halide, PC, or T5 light you need, but how do you compare different halide, T5, or PC lamps of the same wattage? You need a unit of measure that is useful to hobbyists that can be used to tell if you are getting a quality lamp and if you are getting what you should out of your lighting system. Hence... PAR.
Frankly however, when you are talking about Halide and fluorescent technology, watts-per-gallon is still a relevant figure when sizing fluorescent and halide based lighting systems. IMO only 10% of the folks out there really understand it. The other 90% have jumped on the bandwagon because on most message boards, if you even mention wpg, everyone laughs at you. The figure is more a fad than anything else. Yesterday, wpg. Today, PAR. Tomorrow, PPFD.