Why is Wildcat going to show up? There are several reasons. First and foremost, it is fun to introduce characters from the comics to spice things up. Whether you announce The Clock King or The Royal Flush Gang or throw out a reference to Sandra Moonday Hawke, DC fans are going to squeal in excitement.
Secondly, Wildcat is not as high-profile as Batman or Superman, so there's no need to worry about overloading the audience with the same character. When was the last time we saw Wildcat involved in something? His handful of quick appearances on the Justice League Unlimited cartoon? Some episodes of Batman: The Brave and the Bold? It's much easier to bring a minor character like him into this universe for a short time frame rather than a big one. The Flash showing up in three episodes would have let fans down. That is why he's getting his own spin-off series. Not many people care about Ray Palmer outside of the hardcore dedicated fans, so we won't be seeing an Atom show. Wildcat is in the same regard.
The third and best reason why Wildcat is joining the crew is because he has personal ties to one character: Black Canary. He is responsible for training her to become the bad-ass that she is. In the Arrow universe, this character has been split in three different ways. Instead of Dinah Laurel Lance, we have aspects of the heroine passed onto Laurel Lance (Katie Cassidy), her mother Dinah (Alex Kingston), and her sister Sara (Caity Lotz). Sara operates as the vigilante The Canary, having trained as a member of the League of Assassins, while Laurel is not quite the action star that she will eventually become.
This is where Wildcat will come into play. Not only is it pretty much written in stone that Sara is going to bite the bullet and die at some point in this series, but Laurel will take up her mantle and become Black Canary as she's intended to be. To do that, she must go beyond her basic self-defense practices and truly become a fighter to hang with the likes of Ollie and Team Arrow. The description of Wildcat's character confirms that this is the direction the series is going, stating: "Grant will be a former boxer who now runs a gym for underprivileged youth. A man with a mysterious past, he will play a pivotal role in Laurel Lance's arc this season."
There is one other aspect to point out here, and that is the age of J.R. Ramirez. Normally, Ted is quite older and a veteran that is somewhat of a father figure to Black Canary and a former member of the Justice Society of America (the precursor to the Justice League of America). In Arrow, he seems to have been bitten by the "everyone has to be younger and sexier" cloud that CW hovers over the production of all their shows. That's not necessarily a bad thing, as some things are open to interpretation and can possibly be changed for the better. Just look at how they have handled Thea Queen and Roy Harper instead of going with Speedy being Roy Harper and then eventually the HIV-positive prostitute Mia Dearden. Thea "Speedy" Queen has a much stronger connection to Oliver and even acts as a connection point between Oliver and the Merlyn family, who, too, have a stronger backstory rather than just being some singular guy Green Arrow fights. There's a chance a good story can be told here with Wildcat being a possible romantic interest for Laurel, similar to how it is rumored that Ray Palmer will have his eyes on Felicity Smoak. After all, as much of a playboy as Oliver Queen is, he shouldn't get the girl all the time, right?