This tracker was one of the most popular music composing applications on the Amiga. Chances are the music from the good old games you remember from your Amiga days, were made with Protracker. (For example the music in Superfrog... still a groovy game, isn't it?)
It was coded by Cryptoburners in 1992-1993. It had only 4 channels, but it was enough for most people. As for myself, I was used to Equinox on the Amstrad CPC, which had only 3 channels. ;-)
The later versions of Protracker offered mixing. You could mix a sample by making a one-pattern track, and shrink 4 channels to 1, having 3 channels empty again. I used this feature quite alot in releases 9 to 21 (except no 15).
In my opinion, Protracker offered the most features compared to other trackers at that time, such as sampling, reverb, reverse, cut/paste, mixing. But as time went by, users began to feel the increasin need for more channels.
The Amiga platform did offer multichannel trackers, such as OctaMED Sound Studio and Art of Noise. I did try out OctaMED Sound Studio, but being used to the Protracker effects, I was simply too lazy to learn a whole new set of commands. Art of Noise used the same shortcuts as Protracker, but it was simply way too expensive for most people, at a price of 79 DM.
So I began looking into Fasttracker II.
Protracker mods are still quite popular today, but mainly for nostalgic reasons.