Simply put, the language of business reporting is a general ledger. If you don't adopt a universally accepted system of managing your business, how would you communicate those results to anyone, in a way they could take seriously?
If you have a table saw in your garage and make a few hundred bucks a year cutting boards to length for friends and family, you probably don't care about that, and a single order book showing who still owes you might be enough. But even then, how do you report your income to the tax authority (you haven't mentioned a country).
What if you want a small business loan? How will you communicate your business results to the bank if you don't 'speak their language'?
But more than that, there is a reason this system is universally adopted: because it promotes cleanliness and allows for a translatable understanding of what is actually happening in your business.
If you don't record your incomes and your costs differently than how you record your capital investments (computer equipment, or whatever), how will you know if your business is profitable? If your business is basically just consulting work, that may not matter, because your costs are almost Nil - but even then, wouldn't it be nice to have a tracking of what your fuel costs are driving to client locations, so you can decide whether you need to charge a premium for out-of-towners?
The point is that good information systems support decision making. Without information, you need to go off of your 'gut', and most businesses don't have the insight to be able to do that accurately, even tiny businesses.