Aid is for when the cleric has already performed a full day’s worth of healing.
The appeal of Aid is that it can be used by everyone who wishes to use a purely mental action, and character’s with no spell slots or uses left in their daily allotment, can still go out into the world seeking good deeds and using their divine connection to make someone feel better—in times when they’re struggling through difficult circumstances. And this lighthearted effect of wholesome altruism is something everyone in the party can benefit from and feel good about doing! =D