Sage is a sentinel, so she needs to play safe, she usually plays behind her teammates when she gets her ult, so sky's healing suits her and do not bring any problem.

When we think of sky's healing there is no logic, because she is a starter, so she'll be in combat more often, there is no time to be standing with her heal in hands, so the sage's healing suits her perfectly, just aim to your teammate and press a button, and return to combat.

( Yes I know my english is bad, I'm Brazilian, I learn english playing games so... that's it.)