The difference is the customer is not affected by electricity/plumbing/furniture that the company is responsible. Customers are affected by the company's website, which is a type of software.
Does your dentist have his diplomas, fellowships, and awards on his shelf? He's in the marketing business.
Does your dentist work on your teeth and makes you suffer and withstand amazing pains? He's in the dentistry business.
Does your dentist have a website? He's in the software business, regardless of if the website is a simple website so you can find his phone number or if the website provides a way to check your appointment date (or some other service).