On some boards the additional phases is to reduce heat as it is cheaper to add an extra phase than it is to add a heatsink (when you consider pure production cost) The downside is since the phases are not 100% efficient, you increase overall power consumption)
It is also largely marketing. People like higher numbers. To the average user, they may thing "I don't know what that is but this board has more of it than the other board for the same price"
When I get these types of board, I run it without the side panel for the forst few hours while running a CUDA accelerated password cracker (mainly because it maxes the CPU and GPU the entire time and gets hotter temperatures than even prime95)
I then use the thermal probe that came with my multimeter, then place it against each of the phase chips (my msi board had no heatsinks on them)
I then monitor the temperatures, and what I found was that each phase had about the same temperature so the power was being shared evenly, especially with a CPU using well over 140 watts since it is overclocked (Phenom II x4 965 overclocked)
So not sure about all other boards but in my case with the msi board, the crazy amount of phases was to avoid the production cost of adding heatsinks to the phases.