It wastes your valuable time and money if you can't measure your return on investment (ROI) once you get a website up and running, so once it is up you need to track and monitor what is response you are getting and how many sales you make.

Who, what and how

Before you even think about developing or commissioning a website, make sure you have a business plan!  Your business plan will have clarified:

The first step in any implementation of online presence, whether it be a new website, social media or even a new product, is to determine what you actually want to achieve from having it.