Eric Lippert does a great job of defining the term Idempotent. I’ve used this term many times both to sound smart and because it is so succinct.
The one place I find idempotence really important is creating update scripts for a database such as the Subtext database. In an open source project geared towards other devs, you just have to assume that people are tweaking and applying various updates to the database. You really have no idea in what condition the database is going to be in. That’s where idempotence can help.
For example, if an update script is going to add a column to a table, I try to make sure the column isn’t already there already, before adding the column. That way, if I run the script twice, three times, twenty times, the table is the same as if I ran it once. I don’t end up adding the column multiple times.