Thank you! I love making it so that each room tells a story of some kind, and there's a definite reason for it to be there. I guess now that it's been brought up, I can call out a few intended spaces. Some of these details I extrapolated from the adventure Murder in Baldur's Gate:
- The Festhall: A seedy tavern where shady business deals and racketeering schemes are made, whilst the bartender and their staff look the other way. To satisfy the demands of high-paying patriars and other nobles of interest wishing to do business with The Guild here, they stock a large selection of rare wines and ales, especially those known to be from exotic locales like the Underdark and The Moonshea Isles.
- A room for Alfrus Manyblades' black-market selection of deadly weapons for sale. He notoriously will sell even the deadliest of implements to anyone; no questions asked. The Guild has been very pleased with his services, and have offered him a permanent setup in the Undercellar to help facilitate their mutually beneficial relationship.
- A room for Vug Gorkul, the refined half-orc herbalist brewing potent potions and poisons in his small, yet well-equipped office. Visitors would do well to not touch the curious plants he grows there, beautiful and unusual as they may be.
- A desk for Nasparl Nintanter, an eyepatch-wearing half-elf selling makeup, wigs, and other elements of elaborate disguises. For those particular with their needs, he invites them back into his "showroom." There they will find outfits in a wide variety of fabrics and styles, which allow their wearers to pass as a Flaming Fist watchman, a well-traveled sea captain, or a foreign noble attending a regal gala, among endless other possibilities. All of course come with pre-stitched hidden pockets in their linings, for whatever you may wish to sneak past even the most meticulous body search.
- A secret passageway used by Ribbons and Nine-Fingers Keene to transport high-value goods and political prisoners between the street level and the Undercellar's inner chambers. The passageway also serves as an emergency bug-out bunker, in the unlikely event that the Undercellar becomes overrun with Flaming Fist soldiers, murderous cultists of Bhaal, or a self-righteous adventuring party harboring a vendetta against them.
Edit: a few minor extra details
The secret passageway is the one next to the vomiting demon pool. You can see where it connects back to the main body of the Undercellar behind a bookcase.
But, I didn't have a specific story in mind with the demon pool? It's present in the reference map from Rats in the Undercellar, so I made sure it was included in this map too. But there's lots of possibilities. The statues could be simple gargoyles, the altar to a god whose worship is approved in Baldur's Gate (like Helm, Tyr, Ilmater, etc.), and the whole setup is primarily a study or place of relaxation. Or, given the underground nature of the worship of Bhaal or Bane in Baldur's Gate, the upper echelons of The Guild could be offering prayers to one of them in secret, and the statues are effigies to that god.