Often when you first have a website built, you may only include what is necessary at the time. It may be that you need to keep the costs down, so decide to build a basic site that you can add to over time.
It is important to remember that your website can quickly become outdated if you do not stay on top of it. Not only can your site be demoted down the search engine rankings if no fresh content is added, but you may also fall behind in what your competitors are offering.
As technology advances and customers become more aware of exactly what they want to buy, you need to ensure that you update your own website to reflect this and offer them the best experience.
It may be that you showcased your products but there were not available to buy online. Overtime, more and more people prefer to make purchases online rather than in person so adding a shop to your website may be a great move. It can also help reduce the overheads of a sale.
Adding functionality can be tricky on a live site, so it is advisable to create a copy of the website and work on that first. Once you are happy it is all working as it should you can then look to swap it over to the live site.