Players are able to send a request to "borrow" a unit from their friends, regardless if they're not in the same server.
This is extremely helpful to new players that recently started playing. If they get stuck in one specific part of a story because their units are not strong enough, or they're missing on a unit they need in order to progress, A Helping Hand would benefit a lot.
(Personal Experience: I spent over 10k gems to get Sakura just to able to beat the Demon Slayer Raid. Even though it's not storymode, some units are necessary to be included into your team just to beat one level.)
ONLY APPLIABLE TO NORMAL DIFFICULTY STORYMODE