My reason: because I'm a guitarist!
Really though, I actually prefer a .110" for the E, and a .040" for the G. In between, I like a 55 and a 75. The reason for these gauges is because they are progressively tensioned -- as the strings get thicker in diameter, the tension rises -- but the problem is no one makes sets like this as far as I'm aware, you gotta buy singles, and that limits the choice severely. String sets like the usual 45-105, they are very unbalanced, with the G being higher in tension than the E. It's stupid. There is no logic to it whatsoever!
Look:
Your average 45-105 set of nickel wound strings on a 34" scale:
G: .045" [NW] = 42.21 pounds
D: .065" [NW] = 49.99 pounds
A: .085" [NW] = 46.4 pounds
E: .105" [NW] = 36.66 pounds
Total Tension = 175.27 pounds
Does it make sense that the D string is more than 13 pounds higher in tension than the E? :scowl:
Custom set made from singles:
G: .040" [NW] = 33.8 pounds
D: .055" [NW] = 35.65 pounds
A: .075" [NW] = 36.8 pounds
E: .110" [NW] = 39.31 pounds
Total Tension = 145.56 pounds
That's how it should be, progressively increasing in tension as the string diameter increases.
Note: This info is worked out using a tension calculator for D'Addario guitar strings. I adapted it for bass, and no doubt the figures are not 100% accurate, but I'd imagine they are close enough to show the relative differences in tension.
Stainless steel strings are higher tension.