If you are really curious why, I’ll give it a shot as it applies to me.
Whichever side you choose, the hand movements are pretty much the same depending on how you hold your reel. Personally, I have large hands & fingers and like to really palm my reels on retrieve with one or all fingers above the trigger, depending on the technique. However, for me that is an uncomfortable casting position for feathering the spool. I usually have my index finger around the trigger with my thumb on the spool when casting.
After any cast, hopefully everyone can agree that your hands come together to meet at the reel. In my case, if I cast right and reel right, post cast my left hand moves to my preferred palming position and my right moves to the handle (2 moves). Most people consider this switching hands and therefore inefficient.
However, if I cast right and reel left, my left moves to the handle but I still need to reposition my right hand to my preferred palming position (2 moves). Unless you have shorter fingers or you are fine with holding a lower palming position, you are likely subconsciously repositioning your palming hand as well. Either way, your hands are going to meet in the middle.
If it feels better to switch or improves your accuracy or consistency, you should definitely switch. As far as efficiency of movement, there is really negligible, if any difference. I cast a LOT with my left hand as I am comfortable from both sides but there is zero difference in my start up retrieval speed.
The reason I choose what some consider backwards is that I personally feel more comfortable reeling with my more coordinated hand. I certainly understand where some may want to use their more coordinated hand to control the power…I make no judgement one way or the other. As far as efficiency of movement or starting the retrieve faster, IMO that alone isn’t a reason to switch. I think the subject of switching hands sounds good on paper but in practice is greatly overblown.