I'm sure others can chime in with what the benefits of Manual shooting is, but I'll give one: if I determine that a given exposure is ideal for a given circumstance, then I don't want changes like for example, someone with a white dress walking by, fudging with that exposure choice. If I am in an Auto mode (whether it is P or Av or Tv), and in this scenario, my camera will change my settings from the ideal exposure I have chosen to, in this case, one that is slightly underexposed.
Much of the time when we're shooting, we're dealing with fairly fixed lighting circumstances. I like that when I edit a bunch of shots from the same perspective, the exposures of those shots are identical, so I can basically tweak one and copy across the 'set'.
I confess that I shoot 99% of my stuff in manual, but I honestly do see good reasons for sometimes shooting in an auto mode, especially when light is changing faster than we can really adjust.