I'm a big fan of "transparent" fragrances. The exemplar, obviously, is JC Ellena and many of his scents, beginning with much of his early work, notably Bvlgari Eau The Vert (I haven't tried his stuff earlier than that one, so I can't comment on it). Other faves of mine that would probably fall into the transparent category, IMO:
-Hermes Eau de Pamplemousse Rose
-Bvlgari Pour Homme (my all-time favorite)
-Eau de Cartier
To me, transparency means fragrances that don't have strong/rich backdrops - sort of the opposite of Chergui or Shalimar or...you get the picture. There are still base accords, of course, but even those are a bit light, a bit airy, and don't give the fragrance a ton of body, per se. They're more ethereal in quality, sort of impressionistic, as a water color painting.
Why do I like them? I'm pretty understated in my fragrance preferences. I wear them for myself, and almost never think of whether others can smell them -- to me, terms like "silage" and "projection" are generally unimportant (not always, but generally). I also think transparent fragrances can more typically do well in hot weather than the richer stuff, and since I live in a rather hot climate, this is an important characteristic for me. Transparent fragrances, again just generally speaking, are often more likely to fall in line with my tastes, as stated above, than the strong, rich fragrances some others prefer.
This is just my take on them. I'm by no means an expert in perfumery, and I'm sure some folks understand transparency differently/better than me. But, for what it's worth, that's my take on them