The main reasons homeowners opt to include a spa is for relaxation and social aspects. Small spaces are more inviting and make us feel more comfortable (e.g. why do we all crowd into a kitchen at a party when the rest of the house is clear of people?). When the water temperature is lowered to that of the pool and the jets are turned off, small children will often spend more time in the spa due to the intimate space.
When a pool has an attached spa, a parent can keep an eye on both areas. It is also cost efficient because the pool and spa share the same filter and heater system. Due to the comingling of water, it becomes much simpler to keep the spa water clean and balanced. A spa does not have to be literally connected to the pool in order to have these benefits, but rather it can be in the same general location. Most clients see these benefits and opt for this setup.