Clock crystal:
This crystal clocks an internal timer (real time counter) which can then be used to realise an internal clock. Especially when a 14,7456 Mhz crystal is beeing used, a clock crystal is important as only then a exact timing is possible. Also the clock crystal is useful at any other frequency. Remark: If the power fails, the realised clock will also as the controller would not run anymore (you will need a battery-buffered real time clock then).
Save money:
If you are ordering a module together with the buffered real time clock option (such as the D071x), you do not need this clock crystal - if you also order a ATMega2561 or AT90CAN128 as microcontroller.
Explanation for experts: If you order the buffered real time clock, it will be able to provide a clock signal to the TOSC1 line (there is a connection on the board together with a jumper to connect this line to TOSC1).
Caution: An external clock signal on TOSC1 is (due to the datasheet) not recommended on an ATMega128. Thus: with a ATMega2561 or AT90CAN128 you may order the board without the clock crystal - with a ATMega128 you should order the board with the clock crystal.
You may use the real time clock option together with the real time counter as the real time clock will be able to provide a clock signal (programmable between 1Hz and 32.678 Khz) for the TOSC1.
But of course you may use clock crystal and real time clock together if you like.