What would be the evolutionary advantage given to mockingbirds by mimicking other birds?

I would think that the two main reasons birds sing is to attract a mate and to claim an area of territory as its’ own.
So what is the advantage of using another birds call?
Do they get some interspecies nooky? To each his own but it would seem biologically unproductive (or unreproductive anyway).

And when you claim territory it is usually from other members of the same species either to protect food resources or to keep competition lower and increase the opportunity to breed. So what it with the mimicry? Sure, I can see it keeping away a robin or a cardinal but does it keep other mockingbirds out of the claimed territory? Do they know it is another mockingbird faking it? And if so, wouldn’t the other birds being imitated?
Finally, do mockingbirds actually have their own call? All I can recall is the high pitched screech they give when chasing another bird away.