There are multiple reasons, actually, not the least of which is that they REALLY enjoy coming. They love riding in the car to and from the office (and sometimes stopping off at a little neighborhood park on the way home for a quick pre-dinner romp), and having the opportunity to socialize with a great many different people. For Tucker, this is something he has always enjoyed. He is Mr. Social Butterfly. The King of the Welcome Wagon. Phoebe, on the other hand, used to be far more reserved. For those in her trusted circle, she was (is) a total love bug, but she did not often extend those same charms to random strangers. Coming to the office three to five days a week has really brought her out of her shell.
I also love the calming influence they have over not just me, but my colleagues as well. They seem to know when someone needs cheering up and can use a dog to pet or play with. Phoebe, as has been well-documented, can always be counted on for comic relief.
Also, having them here gets me up out of my chair and outside at least three or four times in a day. Since, daylight savings time notwithstanding, I often leave the office after it's dark outside, if it weren't for them, I might never see the sun during the week!
My "work/therapy" dogs relaxing in the shade during one of our pilgrimages to see the sun.(This table is such a relaxing place to sit and look at the water, and since no one ever uses it (preferring the similar tables to the rear of the building that have a view of the many sailboats docked at the marina), it has become "my" spot, and I therefore don't feel guilty when Tucker & Phoebe want to jump up on it to get a better view of their surroundings.)