Some of the benefits of trailing are explained in this video. There are others I did not mention, like the low cost of storage and the ease with which you can take your boat home for winter maintenance.
And the benefits of trimarans over monohull day boats are not just limited to their low weight, which means you don't need a big car to tow them. Trimarans have slimmer hulls that mean they are much quicker and will go in a puff of wind, they are unsinkable and there is a lot of space on them because of the trampolines. (Trampolines also reduce the risk of falling overboard.) Trimarans don't heel over the way that monohulls do. This is described as stiffness and means that the sails propel the boat forwards instead of spilling the wind in gusts, as happens in monohulls. I don't know why this is but ladies, on the whole, do not like boats that heel. So this is a big advantage with trimarans.
Compared with catamarans of similar size, say 19ft, they are much more comfortable to sail because you are not scrambling around on your hands and knees, ducking under a low boom. You are able to sit in them comfortably and the boom is at a safe height. And they tack as easily as monohulls do because of their centre-boards, whereas catamarans are difficult to tack.
And when compared to keel boats the benefits are legion!