Whenever you don’t have a CTO. Or a product strategy. Or both.
Save money and most importantly time by reading this: How much does a website for a startup cost?
Look for ex-CTOs of startups in your space / field skilled in technology stack of your choosing. They have the skill, experience and vision to architect your (product development) strategy which you should factor in into your businessplan. Learn on other peoples mistakes paid for by other peoples money 
Look where? Linkedin, developer and startup meetups, twitter, blogs. Once you find one you’re facing a challange of convincing them to drop everything and work with you. And they are picky…
Here’s some more advice on choosing and working with one: How do you hold an outsourced team accountable for the development of the web application?
Also look at our company example and shameless autopromotion – both me and my businesspartner we’re ex-startup founders with our share of ups & downs.
Plenty of know-how to build upon…
Check out our CTO as a service offering to design your product development strategy.