Some LKMs are set up to retain information from one load to the next. This is called persistent data. When you remove one of these LKMs with rmmod, rmmod extracts certain values from the LKM's working storage and stores them in a file. When you next insert the LKM with insmod, insmod reads the persistent data from the file and inserts it into the LKM.
See the --persist option on insmod and rmmod.
Persistent data was introduced in November 2000.