When we used magentic tape as our primary medium to capture basic tracking (and everything else really), it was possible to get a degree of compression from saturating tape beyond 0 dbs. This is the creates the "warmth" of using analog 2" tape that you may have heard about in the past. In the digital realm it is usually bad to let your levels go above 0 db as you run the risk of clipping (which is just distortion and generally considered to be unwanted). Plugins like "True Tape" offer a warming effect give smoother compression that was possible by slightly saturating magnetic tape. Some people feel it adds warmth back to "cold" digital tracks.