It becomes even more critical if the company is running their own mail server or Web server. At minimum, it means lost productivity and, at worst, it means a lost customer. Fixed cost for easy budgeting. There are no hidden costs or usage fees. Performance on demand.
Users often download huge files, or access Web sites with substantial graphics.
If the bandwidth isn't available, there are significant delays, causing significant losses in productivity. That's near T1 speeds for a very affordable price. Easy to manage. Today's economy is extremely competitive. Few companies have the luxury to burn expensive internal cycles on managing their Internet access.