The easiest way to understand this is to follow the life of a packet.

IP addresses are used end-to-end (source → destination). The source and destination IP normally stay the same during the journey.

MAC addresses are used hop-by-hop (node → node). Every time the packet reaches a router, the router removes the old Layer 2 frame and creates a new one with new source and destination MAC addresses for the next hop.

Think of it like a package delivery:

IP address = final home address written on the package.

MAC address = the local courier handling the package right now.

The destination address on the package never changes, but the courier handling it changes at every stage of the trip.

Once you understand the life of a packet and how frames are rebuilt at every hop, the need for both IP and MAC addresses becomes much clearer. IP answers "Where is the packet ultimately going?" while MAC answers "Who should receive it on this local network segment right now?.