When entering a large MTT, in buying a piece of other skilled players you are insulating yourself against the possibility of busting without cashing and leaving with nothing.
If you are a good player and your friend is also a good player, it is very common to swap 10% because if either of you take it down, you both do well (profit).
This is a common reason people 'swap' but based on the principle, I'm sure you can see why sometimes it can make sense to buy/sell percentages.