First, I use Sonar as a songwiting/arranging tool to construct a song to the point of being like a "demo quality"; as a semi-finished product - its a hobby, and I play all the parts myself except for the loops I use
I use drum loops instead of constructing midi drum tracks or using a real drummer. To me its a lot faster to cut and paste a drum track this way, and it sounds like a miked drum set played by a real drummer because it is. I overcome some of the limitations of not having an infinite assortment of patterns and fills sometimes by "de-constructing" the loops and re-constructing into another pattern, or adding an extra snare hit here and there. I also use an occasional conga, tambourine, shaker or other percussion loop since I don't own any of these (and don't have the $) Someday I would like to learn how to make midi drums sound more realistic so that I could construct my own drum parts. I guess that I can justify using these drum and perc loops by saying that I am not a drummer, and I can't afford to pay a studio drummer to track the part. As far as available loops of instruments like guitar or keyboard or whatever, I have not used any of these yet, it seems like it would be hard to find and then integrate them into a project - there are a lot more variables than a drum part. To me it seems like you would almost have to build your project around the loops - the chord changes or riffs or whatever in that key... I could see myself maybe using a little sax riff or something like that to add a little "flavor" if I could find a set of loops with the right feel, in the right key, in the right tempo range, etc etc., again since I don't play sax..
A loop can also be the 10 measure chorus of the rhythm guitar track that I recorded, that I can just copy and paste for the other 2 choruses in the song. It could also be a 4 second recording of my dog barking in a peculiar way that I would like to use at the end of each verse in my song (never get him to that again !) If you havn't used loops, download a demo version of Acid or Sonar, read up and experiment to grok the possibilities. You can build music around loops, but you can also use them to save time or add things to your music that you could not do otherwise...Hell, George Martin utilized loops for the Beatles - they were pieces of tape that he spliced instead of computer files; different technology, different amount of labor, but essentially the same thing. To me loops are one of the tools that make computer recording an advantage over tape based recording.