I almost always do a mirepoix of onions, celery, and carrots sauteed in olive oil as a flavor base for many hearty recipes. I find that the onions and carrots caramelize just a bit for a slightly sweeter (and richer) finish in the final dish. Heat to the vegetables brings out the natural sugars IMHO, and takes the "raw" notes out of the celery. Although I do not know the chemistry, heating of the vegetables also appears to add to the richness as well. I do add garlic, but it is toward the end of the saute so that it does not burn (as has been mentioned by several of you gents).