When a band records a song, the engineer needs to get the different instruments on different tracks in order to mix and edit everything properly. These days, when track-count is not an issue (due to computer recording - in the 1960s for example, The Beatles only had 4 tracks to work with most of the time!), it isn't unusual to have 50 tracks or more, each with a different take for every musician in the band.
One common recording engineer complaint (all the singers are ducking right now) is that each musician wants to hear more of themselves and less over everyone else when recording. This can be good in that they can hear what they are doing better. But it can also be bad in that their performance, regardless of how technically excellent it may be by itself, may not work well in the context of the overall mix.
Here is an article that discusses this issue: http://www.prosoundweb.com/article/top_ways_to_help_musicians_hear_themselves/?utm_source=feedburner&utm_medium=feed#When:20:41:17Z