Many businesses have somebody or some people in the office all day answering the phone, and responding to emails. They are there in the front office sitting in front of a computer. Many of the sales people have a computer in front of them, as do support people.
One thing sales people know how to do is sell product. A chat system on a business site gives the visitor a way to ask a question to the sales person. People do not like to be sold to until they cross a line – web site copy needs to reflect this and stay on the soft sale side of the line for most of their content. But when people are ready to buy soft selling does not answer their questions. A chat system can close the sale.