Because it’s no fun playing in the rain and humidity, and it rains often in Tampa (45 inches on average spread out typically over 106 days per year). Also, there is a very high concentration of lightning strikes that Tampa Bay receives every year during the hot and wet summer months.
Having a roof is less hassle than dealing with rain cancellations. Or players being struck by lightning. When we first moved to Houston (another hot, wet place) an entire high school football team was hit by lightning when they were in a huddle.
Besides, as Bear08 says, if Tampa can afford it, why not?