There are two main reasons why it is important to follow this law.
The first reason is ethical. There is no good reason why people who are disabled should not have equal access to places of business, either as customers or as employees. So long as their disability does not disqualify them for the job, they have the moral right to work anywhere and do business anywhere.
The second reason is more practical. Failure to obey this law can harm a business. A firm that disobeys can incur fines. It can lose out on potential customers and/or employees who are disabled. It can see its reputation decrease if people find out it does not make an effort to accommodate the disabled.
Thus, there are both moral and practical reasons why firms should obey this law.