reduces static electricity and prevents frizz and split ends. helps give a silky texture and if you have long hair it prevents matting. if it sounds like too much trouble to pick a seperate conditioner, Head & Shoulders 2-in-1 is the go-to product imo.

IIRC... Shampoo is soap-based, and cleans stuff out, but leaves it clumpy and tangly. Conditioner has totally different properties, it's oil-based so on a molecular level it makes hairs less likely to stick to each other.