Normally it would be if someone had a bad database design (but not always).
If someone has two tables that are identical (or very very similar) they
would need to pull data from more than one table. In most cases, these two
tables could have been combined into one with a "type" field.
Example: A table is created to store "transactions". At the end of the
year, a new blank table is made and all transactions start going into the
new table. tblTransactions2003, tblTransactions2004, tblTransactions2005,
etc. This is a case where it would be better to simply include a date and
keep them all in one table.
Example2: From yesterday - One table contains vendor names, numbers,
addresses, contacts etc. Another table contains customer name, phone,
address. The poster wanted to create a master phone list to print for some
department. A Union query allowed them to pull all names and numbers to one
list.
Hope that helps.