Well, they're a visual manifestation of the soul unique to Reapers. Generally, they're a way of showing power at a glance, since size roughly corresponds with strength. In more recent days, they've become a signal of a person's position within the office politics of the afterlife.

[Their purpose, in any case, is to be seen, but not for reasons as simple as pure aesthetics. As for the second question, he just shrugs.]

I haven't had access to the Metaverse for many years, before arriving here, and I don't particularly feel the need to fly around Mementos. It's cramped enough as it is.

[Also, he can line-of-sight teleport.]