Switches operate at layer-2, e.g. ethernet, and they don't get involved with layer-3, e.g. IPv4, IPX, IPv6, AppleTalk, etc. This allows switches to switch traffic at layer-2 for any layer-3 protocol.
ARP is used by hosts to translate a layer-3 address to a layer-2 (MAC) address, so switches don't use ARP, nor are they even aware of the layer-3 addressing.
If a host doesn't have the layer-2 address for a particular layer-3 address in its ARP table, it will use ARP (broadcast) to discover the layer-2 address for that layer-3 address.
A switch has a MAC address table, not an ARP table like the hosts (except where it is a host for management purposes, but that has nothing to do with the switching function). While an ARP table can look up a layer-2 address from a layer-3 address, a switch MAC address table will look up a switch interface from a layer-2 address. Many people get this wrong.
If a switch doesn't have a layer-2 address in its MAC address table, then it will flood the frame to all interfaces, except the one where it entered the switch.
Switches will broadcast any frame with the broadcast layer-2 address to all switch interfaces, except the one where the frame entered the switch.
Multicast, at layer-2, where switches operate, is a form of broadcast, and multicast frames are treated like broadcasts. This has been mitigated by IGMP snooping in many new switches. This allows a switch to snoop on the IGMP requests by hosts to a multicast router. A switch with IGMP snooping enabled will learn and build a table of which interfaces have requested to join which IGMP groups, and it will only send traffic to those interfaces for that multicast group.
None of the switch behavior has anything to do with ARP or ARP tables in hosts.
Not every device has a globally unique MAC address, and I'm not sure why you think that. Some layer-2 protocols (in particular the IEEE LAN protocols) use MAC addressing, but some use other addressing or no addressing at all. Of those that use MAC addressing, some use 48-bit MAC addresses, and some use 64-bit MAC addresses, but all are LAN (layer-2) protocols.
A vendor can buy an OUI from the IEEE, and they can assign the MAC addresses in its OUI as it sees fit. A vendor that makes ethernet, token ring, Wi-Fi, etc. NICs can reuse the same MAC address on all those. Some vendors reuse MAC addresses in different regions of the world.
If you mean the reason for the U/L bit in the OUI, that was part of the original specification. End-users can set that bit and assign MAC addresses in a way that works for them.
Best Answer
One key use for per port MAC addresses on switches is for Spanning-tree BPDU's. These are Layer-2 multicasts with source MAC address of the egress switch port.
I would have to brush up on other Layer-2 protocols such as TRILL and SPB, but they might also take advantage of a per port MAC.
Does that help?