Employees work for your company, so they go into the employee database. But they might also buy something from the company, so now they have to be put into the customer database. Now we've got that person in three different places. Some people only have one or two phone numbers, others have half a dozen. Addresses are the same.
Companies have satellite offices across the country and sometimes more than one part of the city.
Customers can have one address for delivery and another for billing. In some database applications, we find that parties share these contact mechanisms. Databases such as those to track church memberships, student enrollment or attendance, apartment complex tenants, etc., often deal with families where the members share at least some phone numbers and addresses. This happens less often in most business settings, but can still crop up. And then there are the relationships between parties.