The top reason homeowners should think about including a spa onto their existing pool is for relaxation and social gatherings. For the same reason we gather in kitchens at a party small spaces are more inviting and intimate. When the jets are off and water temperature is lowered children opt to spend more time in the spa because it is more intimate space.
In the same instance an attached spa allows parents better ability to keep both eyes on the pool and the spa. Their is also a cost benefit as an attached spa would share the same filter and heating system as the pool. when combining both a pool and spa it becomes simpler to keep the spa and pool water balanced.
Most clients see the benefits of this combination but a spa does not have to literally be connected to the pool but just needing to be in the general location.