Some people will enter a 'short' date indicating they will travel asap (just as Mark mentioned with their 4 week indicator)
Other people may have a number of reasons to indicate a longer date (say 3 months more in the future) just to allow time without using up visa validity.
The Spouse Visa will be issued with validity for 33 months.
The important issue is to be sure that 30 months living in UK from date of arrival can be achieved.
If not then it means paying out for an additional FLR unecessarily