I think people have various reasons for switching resorts. I'll share our personal experience regarding a split stay.
Originally, we had booked a stay at Port Orleans. As a 'special treat' we wanted to try Animal Kingdom Lodge. Booking the entire stay at AKL would be WAY above our budget.

So, for a little less than what we were spending for our entire stay at Port Orleans, we were able to book the first 7 nights of our trip at POP century, and the last 4 nights at Animal Kingdom Lodge. The first part of trip will be pretty busy and "park" oriented. The second part of the trip will be a little slower and we will spend more time at the resort. We really like POP, but are REALLY looking forward to a relaxing stay at AKL.
I don't know if I would be brave enough to split a resort stay on a trip of less than 7 nights, but there are lots of people who do. I think reasons people may split their stays are: resort room availability, resort discount availability, travelling with other families for part of the trip, or just the chance to try different resorts. I'm sure there are a ton of other reasons too.
Hope this makes sense!