You'll have no choice but to visit this building if you play the game. You'll have to meet Mayor Tortimer when you are getting started. That is where he sleeps most days. Must be a cushy job, huh?
Pelly and Phyllis
Pelly will be at the counter in the daytime and Phyllis will be there at night. I wonder who you'll like better. Both of them will be at the windows at the same time if you are playing wi-fi.
Things you can do at the Left Window
Environment - You can ask about the environment when you are trying to get the perfect town. You'll hear tips on how to improve your town. There is a little about this in the flowers guide. When you finally get the perfect town they will say your town is in "Fantastic shape!"
Complaints - You can also make complaints about your village neighbors. You may wish that complaining will cause a villager to move away, but alas, it really has no effect on anything. Have fun blowing off some steam though. Haha
Town Fund - At the left window you can also donate to the town fund. This would be the place to give bells to earn a bridge (200,000b), a fountain (500,000b), a choice of a lighthouse or a windmill (1,000,000b). Everyone can work on the town fund collectively, but friends from neighboring towns cannot donate to your fund. Of course they could give it to you to donate. Watch out because when Phyllis is there at night she will give you the option to donate to her account instead of the town fund. Such a grumpy and sneaky little clerk she is!
Things you can do at the Right Window
Mail - They'll help you with sending mail. You can send mail to anyone in your town or to a wi-fi friend's town. You cannot send mail to a wi-fi friend until you have actually visited their town or they have been to your town. Why do they always say they can't guarantee the delivery? Due to problems with wi-fi it may never get there. I've never experienced that, however you might not want to send rare items through the mail that cannot be re-ordered from Nook. Check the mail guide for more info.
Saving Mail - You can save mail also. There are 16 pages with 10 spots on each page where you can drag mail to and keep it indefinitely. Why save letters? Maybe it is sentimental or interesting. Another useful purpose of this function is extra storage of items. You can put an item in a letter and stick it in the saved mail slots and Voila! Extra storage! The only drawback to this would be that you can't view what the item is until you drag the letter back into your pockets and remove the item. That can be a pain but you may be desperate for storage.