Leto, the
userPresets.package contains all of the the custom patterns you had created and/or used in CAS. That would include any patterns made in CAP, downloaded from the exchange either individually or contained in a lot, object or Sim and also includes any color changes you do to the patterns supplied with the game. So each time you change your Sims clothes, changed a color for flooring or built a house, anything having to do with CAS patterns, it is saved in this file. After awhile it can cause lags and other problems.
I sort of tested it out when I went on a building splurge last winter. I constantly changed the colors and patterns for the walls and floors. Each week my young Sim couple had their entire home and furniture 'remodeled'. They had new clothes each week too!
I started to notice slower play time and kept checking this file's size when I quit the game. I kept thinking the kb's were too low compared to my cache files, but I knew what the file contained and what it did (I also had read this file could be deleted as it would regenerate when a new game started).
So I saved my entire Sims 3 file (just incase) and deleted the
userPresets.package. I loaded up my game and could tell the difference right away. I did not get the lag I had before when my home was loading (used to be all white and then would slowly appear). CAS also loaded much faster as did the build and buy modes
So now I normally delete this file along with the cache files when I quit my game for the day.