Winniebago -
Can an A318 get out of LCY at MTOW?
Can an A319 get out of JER at MTOW?
The answer is 'No' in both cases. And yet these airports are used by these aircraft. Why?
The answer is that at low weights, aircraft are capable of using runways which they wouldn't be able to use at MTOW or MLW. It depends whether the airline can reliably get in and out while carrying a viable load and enough fuel to take the passengers where they want to go.
Dash 8-Q400 out of SZD at MTOW? No.
Dash 8-Q400 out of SZD with max passenger load and enough fuel for AMS or CDG? No problem.