What is the point of batch sparge compared to no sparge?

I've been reading about people who have cooler mash tuns. I guess they mash with a small volume of water, run it all off into the brew kettle, then dump the rest of the water needed to get the correcyt pre-boil volume into the mash tun, and run that out as the sparge. But this seems like extra effort compared to just adding all the water at the beginning of the mash and using all of the water from the start. Is there a chemical difference or do people just do the 2 runnings method so they don't need a giant mash tun (I guess you would need 10 or more gallon mash tun to do no-sparge for a 5 gallon batch)?

Getting the proper grain to water ratio is important.. I can't remember the exact effect, but if I remember right, too thick, and you risk running a poor efficiency, and too high and you risk ruining conversion from starch to sugar.

From my understanding when people talk about no sparge, just means they mash enough grain at the right grain:water ratio to come out with their full pre-boil volume.

Well you get a slower mash and more beta amylase with higher water/grist ratio.

Also if I am doing a larger OG batch, even with a 12 gallon MLT, I cannot fit enough water in there with the grain to do no sparge on a 5 gallon batch.

It's probably the efficiency aspect primarily. The mash tun size is a consideration, but that's easily overcome with a bigger, rectangular cooler. There is a great thread on here, search for "I love no-sparge brewing". It'll answer your questions.

I've been doing BIAB where I squeeze the bag but otherwise do no sparge. This is how I was taught. So if I am getting correct pre-boil volume, then I must be using much more water to grain ratio than people who use mash tuns. What is the effect on taste from this?

You fermentation will tend to attenuate more so your beers end up drier than if you mash at, say 1.25 quarts/lb.