I've never seen any media (post, video etc.) covering this tiny but useful idea for these 2 ults, so I thought I'd explain it. Yes it may seem obvious to some people, but I think it's still worth sharing.

Ever wondered why that Jett keeps jumping around with her knives out? Why that Raze continues hopping, rocket in hand after double satcheling?

It's all to do with crosshair placement. Specifically, the enemy's.

Usually, a player would hold their crosshair around head level. Jumping changes your head's elevation as well as makes your movement less predictable, potentially giving you an extra half second as the enemy has to adjust.

This is a horrible idea with guns due to jumping weapon inaccuracy, but this does not apply to Jett's and Raze's ults.

That extra half a second matters more the better the players are, as time to kill gets shorter and shorter, every little advantage over your opponent matters.

Hope this was helpful!