As a single business start-up it is most likely not to have financial ability to employ everyone you will need to run the business, at that point the option that most likely comes up is looking for someone who has that skill to become a partner in the business.
At what point do you need a co-founder?
Capital is usually not the issue with everyone, some people have the ability to raise sufficient capital for their business that could even last up to a year without touching the companies fund but even at that, there are some skills that a company needs where partnership is always the best option.
Let us consider the need for a developer in an organization, for a skill like this I believe that the best way to get a good result is by working with someone who has a direct link or direct base with the company and that person will hold the position of the business partner. This idea is important because the job or duty of a developer involves so much detail and the CEO or business person might present the idea in a way he wants it to be, but the developer who knows better will be able to challenge the ideas of the CEO and present a better and more convenient version.
Paying a developer or a technician means every directive given by the business owner has to be followed to the latter and this might not give out the best result, but bringing in the developer into the business as one of the shareholders will require that the developer thinks about new ways to add more life and more meaning to the company without necessarily being scared of what the business owner will say, he sure knows what he stands to gain and what he stands to lose so he will put in his best to produce a remarkable result.
The amount requested by developers may be really outrageous and every single change in plan means you need to pay extra charges, so instead of paying extra charges to a developer outside, it is only logical to give it to someone who considers himself to be a part of the team and who is ready to sacrifice all that it takes to move the company to the top just like the business owner.
When starting a business, no matter how much you have at hand it is usually not just enough as unforeseen circumstances keep raising up their heads and those extra cash are very much needed at least until you are able to stand your foot properly, so inviting someone with a high technical ability to become a partner in business is far better than trying to run everything on your own.