The message sent are called a Gratuitous ARP reply which means the machine broadcasts its ARP rather than waiting for a client to ask for it. Think of it like a 'hello I'm here!'
The reason it keeps doing it is to ensure that all clients keep their ARP cache looking at the attackers MAC. Otherwise the original machine could broadcast their gratuitous ARP message 'hello I'm here!' and cause the clients to see their real MAC again.
It also continutiously sends the messages to ensure new clients see it, expiring clients get updated.