I do it so I can more consistently control the viscosity plus the amount of wax and lubing properties in the JPW that is mixed with the LLA. Additionally, to get it to mix with the LLA, heat is required. I never had much luck getting the two (LLA and JPW) to mix and blend well at just melting temperature.
The small amount of mineral spirits (10% or so) seems to help keep the viscosity more fluid, especially when you heat it up as well as making the application onto the boolits a bit easier.