Because
DVC is addictive!!! And the more you stay, the more you want to stay, so you need more points!
Adding on at a different resort is also helpful because it gives you "Home Resort" privileges at more than one resort. Though you only have "Home Resort" privileges (e.g. the 11 month reservation window) for the number of points you have for that resort. For example, if you have 200 OKW points and 120 BCV points, at the 11 month window you can only book a BCV reservation that takes 120 points or less, though if you've borrowed or banked additional BCV points you can use those, too. Hope that's not confusing.
Laura