8 bit PICs: timer1 asynchronous vs synchronous operation
Some operations on the timer don't work when it might change in the middle of a cycle.  This timer is used in conjunction with CCP modules too.  Read the datasheet section on the CCP modules and you will probably find restrictions of when it must be run in synchronous mode.

For example, compare to a particular value can be flaky if the timer value might change in the middle of the compare operation, or might increment past the compare point between checks.