What are the benefits of external bottom brackets? Pretty soon, I plan to replace the bottom bracket on my early 90's Sakae Litage road bike.  It takes a standard, English-threaded bottom bracket.  Right now I think it contains loose bearings--I was planning to replace it with a decent-quality cartidge.  

I see a lot of external bottom brackets when I am looking at my LBS's website.  I have replaced many sets of loose BB bearings and several cartridge BB's.  I know nothing about these external things.  Why do there seem to be more of them for sale than cartridge bottom brackets?  Do they have larger, longer-lasting bearings?  Would it benefit me to consider one as opposed to the cartidge BB's that I have been using recently?

I ride pretty seriously for fun, commuting, and errands... I built this bike specifically for doing really hilly weekend rides.  But I'm not a racer.  

Curious.  Many thanks.
External bottom brackets allow you to have both large bearings and a large, hollow bottom bracket spindle.  A large hollow spindle can be designed to be as stiff as a small solid spindle for less weight.  Smaller bearings reduce the longevity of a bottom bracket, so the typical bottom bracket design allows a narrow range of suitable spindle sizes and bearing sizes - to get the weight savings from a larger spindle you need to sacrifice durability.  Moving the bearings to the outside effectively removes this tradeoff.

There is an additional performance factor in situations where high stiffness is required in that the distance between the cranks and the bearings is reduced, lessening the torque on the axle.

For commuting, errands, recreational riding and other "normal" purposes a cartridge bottom bracket is almost always going to be more than sufficient.  If you are particularly concerned about durability, the SKF bottom brackets are engineered for 10 years / 100,000km of riding, but for the price a $25 Shimano cartridge will usually last what feels like forever.