Quite honestly, that was just how I was taught, didn't matter to me at the time, seems to work so there we go.
Yes, it's more a western thing than an english thing, but to be fair - I think "Tee-Rot!" and "CAN-ter!" is ridiculous, so the english riders and I who share the same barn tend to razz each other about it a little. lol.
It's just easy for me, it's an easy noise to hide from judges when I'm showing, and since I use legs for so many different things....The vocal cue helps.
For example, not always when I apply outside leg to the hip do I want to lope off. Sometimes I just want to move the hip over. Not always when I squeeze with both legs do I want a trot, for me, squeezing also means either to round up, or drop the head lower. Now, if I do either of those things and pair it with the cluck/kiss, THAT means trot or lope.
As a general rule too, it's acceleration more than anything. I also use the cluck and kiss in the spin. I cluck in rhythm with the outside front foot to establish cadence, and when I want some real speed, I'll kiss in one long continuous smooch. (Don't ask me why, I think I just started doing that because once you get faster, it's too hard to cluck in time with the front leg. So has to be just general laziness on my part.)
Regardless, my horses understand it, I like to think of it as a gear shift. Legs are the clutch, and the cluck is second gear, kiss is third gear, and if I want any real speed I'll push in the clutch again and continue to kiss.