Using sound object oriented software development techniques is fundamental to any software system but in particular to start-ups. Writing clean code thats easy to read/change is fundamental to maintain/develop for the business to grow. Following the principals of SOLID allows the system to adapt to meet changing needs. For example if you choose to use Stripe as the payment gateway but then wish to switch to another provider the code/logic has to be contained within its own domain(classes, api's, etc). If this logic is mixed into the general business logic then it's more difficult to extract later on. This is just one example I have experienced when working in start-ups. Certain services start out cheap but once you grow and need more volume the cost's increase significantly(they move you from free/startup package to SMB but you might be some where in between). You want the ability to swap these external services out easily. You do not want to be locked in to a particular provider/service and held to ransom.