If you were a roof, the last thing that you would want is water laying on you all day. Your roof was not meant to be a swimming pool.
There are a lot of ways to drain water from a roof. There are roof drains, “K” gutters, built in gutters, overflow drains, half round gutters, scuppers and downspouts. Heck, sometimes it’s OK just to let the water run off the edge of the building. 
You do not want to have water “ponding” on your building and here is why:
1. It shortens the life of your roof.
2. Makes it very difficult to make repairs.
3. Adds a lot of weight to your building.
4. May void your warranty.
The National Roofing Contractors Association (NRCA) states that
Good roof design practice dictates that there be no ponding water 48 hours after rainfall, under ambient drying conditions.
Major roofing manufacturers reinforce this by making it a part of their specifications. IF the roof does not have proper slope, there are ways to add slope through tapered insulation systems, but that’s a topic for another day.
