Trub is comprised of the following: - Proteins from the grains - Break - Live yeast - Dead yeast - Bloated yeast
Some brewers rack their beer off of the trub in order to prevent autolysis. This happens when the yeast begins to die and produce off flavours in the beer. Some brewers claim that this risk is overstated, as empirically people have left beer on the same yeast cake for months with no autolysis issues.
'Dumping on the yeast cake'
Some brewers just drop their wort on top of an existing yeast cake. What they do is rack the beer off the yeast cake and splash the wort on top of the yeast cake. Debate over this is similar to that for racking- it has been argued that this risks autolysis, though those who do it argue that as long as it isn't done for too many generations, it's not a risk. Doing this can also risk overpitching since by the time a fermentation is finished the yeast population is quite high.
See: Yeast Farming
It is possible to farm the yeast from the trub. However, the yeast needs to be washed in order to separate the trub solids out and get the pure yeast.