If you don't have an "About" or "About Us" page easily available and offered as a link in the header, side bar or bottom links then you could lose a lot of business. People want to know who they are dealing with. They need to know how long you've been in business and what your background is. Basically, you need to explain to them why you're the business they should choose.
If you're an ecommerce store, tell them how long the store has been running and how you came to sell these products. Give them some company history. If you are offering a service, tell them why you are qualified to offer this service, briefly include your past experience and maybe what you like about the work you do. If your website is a blog, tell them about yourself and your interests.
The About Us page not only explains why the customer should choose your shop, it also humanizes you to them. By telling them a bit about yourself and what led to starting the company people will feel a greater connection to you and have more confidence in the work you do.
Anyone can set up a website - that's why a smart online shopper is cautious and does a little investigating. Making sure your About Us page is available, simple and friendly is a great way to let people feel at ease doing buxsiness with your company.