It is a good idea not to immediately remove casualties from the battlefield during the Combat phase, but to temporarily place them next to their unit. There are two reasons for this:
Firstly, when it comes to working out who has won the combat, you will need to know how many Wounds have been inflicted this turn. This can most often be quickly determined by counting the number of models removed as casualties.
Secondly, models removed as casualties before having a chance to attack, and models that stepped forward during the current phase, cannot attack. Therefore, the number of casualties inflicted on a unit will often reduce the number of models able to fight back. By placing casualties next to their unit, you can tell at a glance how many models have been removed from the fighting rank.