they were intended for low visibility personal protection or use with ditch kits. it wasn't anything intended for "faster reloads" or whatever. it was because you physically couldn't wear significant webbing due to other things you had to wear like a wet suit, local clothing, et c. and the rest of your kit was in a bag or something. if things became messy, you strapped up. if things kept cool, you had a ready spare mag without figuring out where it was hidden or if it's in a pocket and fell out, et c.
you also had this style which was awkward as all hell, nobody used this that i'm aware of. most people i worked with used the tape "V" method rather than purpose-made couplers, and even then it wasn't super common. most forces i encountered with AKs did not have bandoleers: you were more likely to see a lightly geared guy with an AKM with 4 mags in the chest rig, then one in the gun and one taped to that one - that gives them 6 mags plus some grenades and maybe one or two other things about that size. great for being light on your feet, and you could always just ditch the taped mags if they were a hassle.
for the US, our bandoleers could be used with clips, 20 round or later 30 round magazines depending if you broke the limiter string or not. later in OIF and OEF we had the magazine bandoleers for SAW and M240 gunners that were also used by infantry sometimes. imho i prefer the older string bandoleer vs the shoulder pouch design.