what does a power conditioner really do?

All things considered, I'd rather have unconditional power...

Oh, sorry.  Wrong subject...

Carry on...;)

Unless your mains is awful then don't bother as there is always an element of compression when using a conditioner.
If your mains is ok then choose a very high quality distribution block. 

Syntax said: It transforms cheap power into expensive power  .

Actually, it transforms expensive power into more expensive power. 

In theory, these devices are designed to remove RFI/EMI noise from one's system. Some products are better at this than others. Much will be determined upon your locale.

As with any audio product some are better than others(conditioners are no exception to this rule ) and a hierarchy is created. I can 2nd cousinbillyl's assessment of the BPT  balanced AC  transformers /conditioner. It is a superb device that improves sound quality across the board. I have 20 amp dedicated lines, the BPT is a step above the dedicated outlets. 
Charles,