Originally Posted by Diplomatico

Yes, that. You have an upgrade the first night - then when you get there - you ask them to consolidate the reservations so you don't have to change rooms.

Or....your first night reservation there are no upgrades available. But nights two and three there's upgrade availability. If you had a three night reservation, and the first night was unavailable for upgrade, you wouldn't get upgraded for the duration. With three separate reservations, you have three shots at an upgrade instead of one - then you adjust the reservations accordingly at the front desk upon arrival.