You can mount your router directly to a table top but there are advantages to using a mounting plate; First and foremost is the ease of making adjustments. By lifting your router out of the table it is much easier to use wrenches for changing bits and eyeball adjustments in depth. Another consideration is portability. Some members use a large table at home and have a smaller table for use at jobsites. Plate mounting means you just drop the plate/router into position and you are ready to run. Some plates have removeable rings for different opening sizes so your work is supported properly and yet have clearance when using larger bits. Most mounting plates also allow you to use guide bushings and starter or safety pins for following a template. Aluminum or phenolic are both fine as long as they are flat.(The exception to this rule is the Rousseau mounting plate)