Why is PEG important for efficient yeast transformation?
http://www.protocol-online.org/biology-forums/posts/26634.html  

I found this link - it says 'we don't really know'.  It says PEG binds DNA, I assume shielding the membrane from its negative charge and allowing internalization to happen.  

I would guess that the amphipathic nature of PEG, being partly hydrophobic, also helps soften up the membrane.  Interestingly, if you increase the PEG concentration beyond the limits, it decreases the efficiency of the procedure.