It puts an upper limit on how much you need to grind something, and means that you don't need to refarm gear of a different trait when things inevitably get changed.
Say you get a training VMA staff on your second run, well now you can just retrait that to what you want instead of having to grind out another 200 runs for the staff you want. It also makes Nirnhoned dropped weapons possible, which is nice.
As far as having to know the trait, it takes like 2 days to get the first 3 traits researched on a weapon, so it isn't a huge time investment.
Hell you can just research the trait on a character without any craft experience and have access to it pretty much immediately.
I read so many posts about transumtation and how hyped people are which I dont understand.
Sure, changing the trait on gear is a useful feature, but how often will that happen? I dont think, people have the need that often to transmute. Or am I wrong?