Why do we need the labels of "Liberal" and "Conservative"?

The way I see it, generalizing politics with just two polarizing terms undermines rational-choice. Most people I've come across vote according to what candidates label themselves as, and not their stances on specific issues. It seems that people are so eager to choose a side and defend that side, even at the cost of logic and reason. Using terms like "liberal" and "conservative" puts the blinders of bias on most people, keeping them from looking at the big picture. Instead of considering whether you agree with a person's logic or not, many people only consider whether they are liberal or conservative. I would simply like to know if it is necessary to have such labels when they obviously narrow people's views.