Well, there are many reason to buy an aluminum trailer vs a traditional steel trailer. Firstly Aluminum is lighter by 35- 45% which means your payload is going to be higher than a steel trailer with the same axle rating. Generally though most aluminum trailers have a lighter axles than their steel counterparts but are still able to offer the same payload. This equals a less weight to haul (better fuel economy) and your trailer is easier to move around when hitching up.
Another big advantage of aluminum is that it does not rust nor will you have a rotting wood deck to replace (considering you purchased an aluminum decked trailer). A simple acid wash brings the shine of the trailer immediately back.
One of the main misperceptions about aluminum trailers is that aluminum is not as strong as steel. Actually pound for pound aluminum can be two and half times as strong as steel. The extruded shape and type of aluminum used provides a higher tensile strength. Finally if aluminum was weaker why are airplanes made of it?