I try to do almost all my outdoor shooting with Nikon Speedlites. This should be quite obvious to anyone who reads this blog. However, strobes can not always provide all the light you need. (Unless you can stack a gazillion of them together). Some cases where a strobe will not cut it are shooting against a harsh sun, lighting huge deserted interiors, having to use large diffusers outdoors, and more.
Those are just the types of scenes where I use a studio flash… Hey there! waitaminute!! You said shooting outdoors. Are you pulling my leg with the studio flash thing?