When you tie a bowline to another bowline, you risk the two loops sawing on each other and actually chafing through! To prevent this, all you need to do is pass the line a few times through the loop.
This will take any movement out of the system and allow your two lines to hold securely without the fear of chafe.
We do this to tie the bow line of the dinghy to the painter. The painter has a bowline tied in it and we simply tie up to the loop. Having these wraps means that we can sleep easily without worrying about the dinghy sawing through its painter and drifting away quietly into the night.