The Tower of Commerce was the capitol building of the Ferengi Alliance, the largest and tallest building on Ferenginar. From the Tower's Chamber of Opportunity, the Grand Nagus presided over the entire financial empire of the Ferengi. And, in the Chamber of Petitioners, the Nagus dispensed financial advice to his subjects.
The Tower also housed the offices of the Ferengi Commerce Authority. Liquidator Brunt had his offices in the Tower of Commerce until he was ejected from his job in 2373. (DS9: "Family Business", "The Magnificent Ferengi") Quark also briefly worked in the Tower as first clerk to Grand Nagus Zek. (DS9: "Ferengi Love Songs")
On Ferenginar, some criminals and financial deviants were taken by the FCA to the top of the spire of the Tower of Commerce and shoved off to plummet into the Sacred Marketplace, a full forty stories below, where people bet on where the victims would land. (DS9: "Bar Association")
Interestingly, the first mention ever of the Tower of Commerce in (DS9:"Prophet Motive") was overdubbed as such, but both Quark and Rom call it the "Temple of Commerce". The English subtitles also designate it the Temple of Commerce.