there are two big arguments for a denser layout, notably you move your hands less, which means you can type faster, statistically speaking. It makes it easier. Generally you see typing speed track roughly with this over time.

And since you move your hands less, it's ergonomically better for typing, so you get less strain, you have better ergonomics in general, you can type longer, and even faster since there is less strain.

Different layouts optimize for different things, some optimize for efficient roll combinations, some optimize for switching between hands as optimally as possible. Some don't really do any of that (qwerty) which also have a significant impact on typing.

Layout isn't really about speed, it's about comfort
I've been using modified colemak for like a year now and good lord it's so much nicer to use: you just place your fingers on "arst neio" in my case and then 80% of the keys you actually use on a regular basis are within a tiny finger movement to reach.
And then there's the fact that you're almost always using a different finger for the next letter, suuuper smooth typing experience.

Even the keyboard design itself can effect typing results. Like typing on a really good mechanical keyboard is more comfortable than a shitty chiclet keyboard.