Aid introduces some new concepts into 5th edition that put it in the unique position of being a very versatile, highly contextualized spell.
Aid has been tweaked so that its effects can be multiplied and added to any other effect from a different school of magic. So now the spell acts as an amplifier to make your spells more powerful – and you save yourself 2 slots on your character sheet while still getting this amplification! It’s an appealing idea for players who have always wanted those status-affecting spells (like the Fly spell) to engage enemies in melee, as well.