the higher the gauge of string the lower the note is.Bass strings are really thick.At a certain tension the string rings out as a G,but you could probably tune it to F or F# at a lower tension, or G# or A at a higher tension. This goes for all strings(except different notes obviously).
Some people like to use slightly thicker strings because of tone and volume and maybe feel.Theres also some people that like a wound A string,because they play with quite alot of pressure/harder and the string goes out of tune easily or even breaks.
Im a d'addario man myself .Have experimented with thicker guitar string but just felt they needed more pressure to play them and couldnt play as fast as i wanted.Im also experimenting with CGDA tuning(D'adarrio) at the minute which are much thinner .Its a higher pitched tuning.
Hope that explains alot