On average, Americans wash their hair between four and five times a week [source: Aubrey]. While a quick wash may fix that bed-head you woke up with, shampoos are intended to clean your scalp -- they remove oils, sweat, dead skin, dirt and styling products that build up as you go about your day.
Used properly, shampoo should be massaged into the scalp to remove sebum, the oily substance our bodies produce to help keep our hair and skin hydrated and supple. Always rinse thoroughly and follow with conditioner to restore moisture that's been removed from your hair in the process.
And those two-in-one products? Go ahead and try one -- it'll do just fine if you want to combine steps.