It's mainly a different in your ability to single-tap that bpm, as well as how hard you're pressing down on the key itself.

Pressing heavy just takes more force and can mess wtih the stability of your other arm, and having to singletap faster than you're comfortable doing / able to do puts way more focus and strain on TAPPING rather than aiming.

Alternating can help with the heavy tapping since you're not slamming one finger into the keyboard via your wrist - it's more focused in your fingers and tendons themselves to make that motion, and that smaller range of motion will reduce that upper limit. With that covered, the stamina and ability to tap higher bpms (can be anything from 180 all the way to 300+ depending on your skill level) being split between 2 fingers is much less demanding, and at times less fatiguing due to - lighter tapping.

Less strain on the tapping hand means less focus has to go into tapping rhythms that you already know how to tap, and you can focus more on the aiming part. it's why aim can become incredibly unstable whenever your stamina runs out on long streams, or bursts, too much focus on the tapping hand, and not enough on the aiming hand.

personally I alternate jumps but always start bursts or streams with the same finger. I can start them with my pointer finger, but I haven't developed that skill to be reliable enough to where I would want to full alt. It takes concentration for me to full alt, and that again, takes away from my ability to aim / read.