What for a Kinase Assay?
There's a very simple answer: scale. If a drug company wants to screen a million-compound library to find which ones inhibit the kinase activity of a certain target, they're not about to do a million IPs and a million Western blots. Instead, they'll use recombinant kinase and substrate peptides, with a variety of readout systems to choose from.

Other reasons include not having a good phospho-specific antibody to the substrate, very low expression of the kinase and/or substrate, complicated protocols necessary to activate the kinase in vitro, lack of a good model system, and more.