Because you need to patch up those weak areas.
Let's say for example that a WC practitioner is not good at a technique known as biu sao, which is the most effective tool to use when someone on the street throws a wide "haymaker" type of hook. Instead this practitioner tries using a tan sao. From my own experience, tan sao will not work to block such a punch. Therefore if your biu sao is weak and you want to be able to stop that attack, then you NEED to work on that technique.
Just my humble opinion, of course. I do feel that each martial artist should work on whatever they naturally, instinctively do well in their art. After all, that is what helps them put their stamp on it and make the style "their own."