Having none of the other related doucments to refer to, I interpret this clause as meaning the folloiwng:
1. You have a right to convert your fixed week permanently to a floating week (not sure what season you would be in).
2. You can ask to temporarily (ie. per season) be given a floating week if the existing rules allow it, and you follow those rules.
3. Developer can do any or all of the above at its own discretion (big surprise).
As to why one might choose #1, there could be a multitude of reasons: for example they have a Christmas week and coverted to another religion, or they no longer want to celebrate July 4th, or can no longer stand the spring break crowds?