A lot of people mix it with tobacco in joints/spliffs. It's especially common in Europe, where a joint will typically be 2/3 tobacco and 1/3 cannabis or 50/50 depending on the user.

Virtually nobody in the UK smokes it without tobacco in joints.

There's also the possibility of people in the early stages of addiction trying to satisfy urges throughout the day by substituting with cigarettes (yes it can be addictive...)