Originally Posted by
lungimsam
Does it enable you to do less front chanrings switching when you ride,
I shift my front derailleur at least 10X more often than I did with a triple crank. If I wasn't using Campagnolo shifters where I could transition to the next gear moving from the big to small ring with simultaneous thumb pushes (nearly all Campagnolo shifters will go five cogs smaller at once) I'd probably have thrown the compact on a shelf and bought another triple crank.
When I had my frame turned into a bike in 1996 I opted for 50-40-30 x 13-14-15-16-17-18-19-21 so I wouldn't want to change cassettes based on whether I rode west into the Rocky mountains (where I needed a low gear like 42x28 which would be the same as 30x20 or 34x23) or east onto the plains (where a 13-14-15-16-17-18-19 corncob would be nice). That worked great.
I moved to 9 cogs and added a 23 after Campagnolo discontinued the 13-21 8 speed cassette. I switched to a 50-34 compact after wearing out my big ring, noting that 34x23 was effectively the same as 30x21, and buying into popular wisdom that two rings were better than three.
Eschewing the fully cross-chained combinations 50x21 and 34x14 are the only close gears. For significant time at a threshold pace (if I want to do the same thing tomorrow) any faster than 19 MPH calls for the big ring and any slower than 17 calls for the small ring. Even "flat" terrain with false flats and winds implies crossing that range with ring changes and a five cog rear derailleur shift at the same time to get the next gear (50x21 -> 34 x 15 or 34 x 14 to 50 x 19).
With the triple I could just cruise in the middle ring past 22 MPH on flat ground ignoring the 13 cog and 24 MPH not (the middle ring splits the difference between small and big ring on a double and still has an acceptable chain line + noise there). That's 63 - 103% more power which can be put into overcoming aerodynamic drag before shifting to the big ring (huge).
Slowing down I could spin as slow as 13 MPH without using the last cog or 12 with which covers climbing 4% grades.
In between I'd do a lot of cruising in 40x16 or 40x17 right in the middle of the cassette instead of 34x14 or 50x21 one from the end.
How's it supposed to be used?
The cynical answer is to make more money for the bike companies by reducing the number of SKUs they need to make, stock, ship, and close-out.
Otherwise just like any other gearing setup - with whatever combination results in the least fatigue for a given speed or highest sustained speed and minimizes other annoying factors (more front shifting, more chain noise as you get closer to the end of the cassette or rub on the big ring) where there's a choice between rings.