Too true, Steve ... at between £550 & £600 a time,
most applicants can ill-afford to make mistakes!
with Bluebirdjones ... whose [now] wife, herself came here on a fiancee visa. However, for what it's worth, I was staying in the Phils when MY wife applied for her spousal visa during the third week of January last year. And, since I was scheduled to return to the UK on February 19, we put THAT date on her application form, in the vain hope she would accompany me; in the event, her visa was granted 3 days after I arrived home.
So, to my mind, applicants are being asked THIS particular question purely from the point of view of giving the Embassy a rough idea of when your partner would LIKE to travel, then - presumably depending on its workload around that particular time - she may (or may not) receive her visa before the date specified. Good Luck!