Whether you have a small, medium or large company, each will have an audience who looks at their website on a mobile or tablet.
You may assume that you're website will get looked at during the working day on a desktop, but there are so many hours at either end of the day. What about working mothers combining work with a trip to the park? Some of your target customers only get the peace and quiet to search online in the evening, perhaps cosied up on the sofa busy scrolling on their mobile or tablet. Think of the number of hours commuters spend on the train each day.
A website must capture the feel of your company whether it be a restaurant, a law firm or a design studio and it's got to be responsive too.