The idea it's simple: its purpose is to group channels the way you like: by language, by genre, by whatever. Within every bouquet, you can also add "markers" that are just descriptions: for instance: kids, sport, etc.
All the other channels you don't need will always be available but they won't be numbered. Only channels included in any bouquets are numbered.
With OBH you can also jump to the first channel starting with the letter contained in the remote key AFTER having shown with UP/DOWN the channels in the current bouquet (for example, pressing the "2" key on the remote, as it contains "2abc", the first time you press it you'll select the first channel beginning by "a"; then, if you press again it, it'll go to the first channel beginning by "b", and so on. The same with the other key-s.
So there are many ways to browse channels. But, as I said, bouquets are not compulsory. You could have only one bouquet and, within it, channels separated by markers.
If you are new to VU+, I suggest to gain some experience and become more acquainted with other main aspects, before focusing on bouquets and channels.