Why not just hire someone in-house?
A hire costs salary, ramp time, and management attention before producing anything, and you carry the risk if the motion turns out not to work. Engaging a partner moves that risk off your payroll while you find out whether the motion works at all.
The honest version is that in-house wins once the motion is proven and repeatable. Hiring to discover whether it works is the expensive order of operations, because a hire who cannot make it work still took months and a salary to find that out. Prove it first, then hire against something that already works.