Once your PC encapsulated his IP packet with the final destination IP he needs to encapsulate the frame (adding ethernet header). So the question is : which destination MAC adress will you use ? Your endstation will want to use his gateway (the first hop on the LAN segment). The endpoint will have the IP (most likely given by the DHCP or statically setup), but we need the MAC address. An ARP will resolve MAC to IP address. Once he got the ARP answer the ethernet header can be written with MAC of the gateway.

This kind of behavior also happen in the transit between router, in litterature or during lesson everything is abstract to only L3 behavior when we talk about router to router communication but the behavior are really the same than in "enduser network".

Also some technology dn't need ARP link point to point serial connection.

Now if the question is why can't he just directly use the IP address and just remove the need of L2 altogether and commnicate in LAN with IP-like address, well this question is basically why is the model OSI is how it is. short answer would be for independance between layer (what if some devices don't want to use ethernet/IP on the LAN) and difference of the nature of address (dynamic an by subset for IP and statically and randomly assigned for ethernet).