You don't need one if you have all the knowledge and time in the world, with the network to take advantage of it. So, yes, in most cases you need one.

Now, if you can build a prototype or get traction using your existing team, do it first. You'll get better candidates that way, and be at a place where you have a better ability to choose between them.

I agree with other comments here that state their value add should be obvious.