There are several reasons. Firstly, a Drilled unit that declares a charge whilst in Marching Column can freely redress its ranks to adopt Combat Order after determining its charge range, but before moving, thus allowing it to make a charge move. Secondly, units that are obliged to declare a charge in certain circumstances (those that are Frenzied or Impetuous, for example) must do so even whilst in Marching Column. If they cannot make the charge move, they don't move at all and the charge is failed. This prevents Marching Column being used to avoid declaring compulsory charges. Alternatively, there might be a psychological advantage to declaring a charge with a Marching Column. For example, the unit might cause Terror, or the charge target might already be fleeing. Of course, it isn't easy to set up situations where such tactics can be used and therein lies the challenge.