DHCP issues address leases, not permanent assignments; leases have an expiration time (and earlier suggested renewal/rebind times). So the DHCP client must periodically renew -- if the tenant doesn't renew (perhaps because the DHCP client has been disabled), the DHCP service may lease the address to another tenant.

If the DHCP server hasn't moved to a new address this renewal can be done over unicast using the leased address - however, if the client doesn't receive a response from the server the client state machine will eventually discard the leased address and fall back to broadcast with an all-zeros source address (which is presumably what requires a raw socket).

The DHCP client implementation in question likely keeps the raw socket open for potential future use in this case. A client might be able to close the raw socket and reopen it later (but security folks might also want it to drop the privilege required to reopen the raw socket, and it might be hard to have an ironclad guarantee that the raw socket can be reopened later on a machine that's short on free kernel memory..).