Of my four most carried guns:
S&W 629 .44 Magnum with 4" barrel- the most powerful gun that I can get in a package that is still practical for carry. This gun is carried almost exclusively in colder weather.
S&W M66 .357 Magnum with 2 1/2" barrel- the best balance of power, size, reliability (particularly a contact shot), and shootability that I can find in a compact belt gun. This gun is carried primarily in warm weather.
S&W M36 .38 Special with 1 7/8" barrel- The best balance of power, size, reliability, and shootability that I can find in a pocket gun. This gun is carried as a deep concealment or "grab it for a quick run to the grocery store" gun.
S&W 1076 10mm Auto- The most power I can get in a semi-automatic that is still practical to carry. This gun is not carried nearly as much as the other three and is only carried in cold weather.
Really, it's difficult to pin down any one particular reason that I carry a specific gun. None of my carry guns really do any one thing better than anything else could. For me, however, they do offer the best balance for their intended purposes.