Strangely at least to me I get a lot of people asking after a quote 'does it come with lights?'. I'm still baffled by this question. My first instinct when asked this question is, are other DJs not offering lighting? Although I'm not saying there isn't, I've never found any. It seems to be more likely to me at least that it is important to you the customer, and perhaps you might be looking for a little reassurance. So I can confirm that unless specified (maybe for light sensitive epilepsy reasons) every quote Alt. Entertainments supply will include lights.
But why so Important?
It's actually not that easy to answer, but it has a lot to do with the psychology of movement humans are fairly empathetic as a species, and moods of crowds carry. If a crowd is excited it carries, if crowds are angry that carries too. So the exciting pattern of lights can help crowds get into the mood of having a party. Exciting patterns can have entrancing effect too it's thought this is due to the fire staring nature of prehistorical homo sapiens, and the better the pattern the better the effect.
It's not just the movement of the lights however it's also the lack of light, specifically the lack of light from the house light. This creates a kind of anonymity on the dance floor & anonymity encourages inhibition. Inhibition can sometimes stop us from enjoying a good party as we're fearful of what others might think of us.
Is it always Important?
Don't get me wrong, Lighting is a useful tool but it's not as essential as good music. You can have the best lighting in the world but without good music it will mean nothing. In fact I know of some very high end DJ's who offer very minimal lighting to great effect. As sometimes less is more.