The simple answer is this:

An IP address identifies a host, whilst a MAC address identifies an interface.

That is all.

Absolutely everything else, such as route aggregation, routing policy, and the behavioural differences between layers, stems from this primary distinction between logical and physical.

(Note: it remains a common - indeed, conventional - configuration & design error to bind an IP address to an interface. There is no such requirement and this complicates many networks. I was irritated to learn that IPv6 makes use of the MAC address in self-autoconfiguration)

(ObAuthority: I used to make ISP management systems for a living)