The bulk of people want to compress their snare drum samples, hence they ask this query. A compressor is a common tool used by audio engineers to adjust dynamics by lowering the volume of louder parts while increasing the volume of quieter passages. Compressors can be used as a standalone effect or as part of a larger chain that spans a whole mix. Why would you need to add a compressor to each track when mixing music? It's usually not necessary unless you're trying to moderate excessively loud peaks that come from only one drum (say for example, if you recorded your snare track with too much proximity effect).