You're grouping one major publisher Harlequin (which is part of Harper), Kensington (a smaller but well-established house) and Tule (a relatively new start up, and primarily digital.)Why would an agent submit a book to independent presses? Especially those that will take unagented submissions, like Harlequin, Tule, Kensington... I can hit "send" myself. I'm baffled.
That you think all three are the same is an indication you don't know what you need to know.
As for why I send submissions to Harlequin or Kensington, the answer is easy. They publish good books, and can get them into bookstores. They both have sales and marketing teams who work on an author's behalf. I know the editors there.
And frankly, they pay on time.
Now, the real question you're asking is why do you need an agent if you can just hit "send" yourself.
I don't answer that question anymore.
I don't try to persuade writers of the value of having an agent.
If you have done your homework, and concluded you don't want or need an agent, that's totally fine with me. I'm not going to track you down and yell at you.
If you've figured out you don't know what you need to know, and want some expertise on your side, well query away.
What you can't do (at least as far as I conduct business) is submit to publishers that do take unagented queries, and then query me. When you winnow down the pool of publishers by submitting on your own, you make my job harder. That's not a plus for me.