Trucks have ladders (very, very large ladders). Engines do not. That's the reason!
It's important with all of the high-rises, apartment complexes and tall office buildings in the Turtle Creek/Uptown/Oak Lawn area to have an apparatus that is able to fight tall building fires and rescue trapped people should there be an emergency.
By removing the truck from this location, residents in the immediate area will have to wait longer for an appropriate rescue vehicle to arrive. Station 11 has 2 ambulances and an engine; but it's the truck that makes the station an extra asset to the community.