It's very simple. Singaporeans love food, and are willing to queue up and wait for food that is worth waiting for. Food centers are where you can find a melting pot of Singaporean local cuisine, which in itself is very varied, due to our multi-ethnic, multi-cultural society. The location of the market is also near the city.
Even though it is starting to become a well-know destination for travellers, you should not miss it as the food is truly good. Also, below the food centre is a "wet market", where all sorts of fresh produce from meat, fish and vegetables are sold. In fact my Australian friend who's a chef was so fascinated by this place that he refused to leave.
Monday is a bad day to visit, as most stalls will be closed and resting, because there is no meat delivery on Mondays.
Mon-Su: 07:00-21:00