Why would people use lotion and not moisturizser?

Some people may choose a lotion for very light moisturising.

The difference between a lotion and a moisturiser isn’t clearly defined; a lotion tends to have a higher water content, a lower percentage of butter and oil and is rarely very occlusive.It is often a cheaper, simpler product, without the added peptides, ceramides, vitamins and other cosmeceuticals found in moisturisers and serums. But there is no reason, other than cost, not to add them and a well made emulsion with a high water content is a still a lotion and can be very effective (lotions tend to be 70–85% water; moisturisers 45–70% water).