Seen a few applications/examples which could easily be modified to run a little faster.

Suppose you wanted to record page views, and add to a total every time... I've seen this done before:

SELECT pageViews FROM pages WHERE id=1
UPDATE pages SET pageViews=". $row['pageViews'] + 1 ." WHERE id=1

You can do it with one query, by just using:

UPDATE pages SET pageViews=pageViews+1 WHERE id=1

In a similar style, you can also update a row with values retrieved from the row itself. Suppose you wanted to merge two fields (firstName and lastName) into a single row:

UPDATE users SET fullName=concat(firstName," ",lastName)

You can even do some more advanced (?) maths. Suppose you wanted to shift a set of prices up by 10%, you can do it really easily by simply using:

UPDATE products SET price=price*1.1 WHERE id=1

This is just the tip of the iceberg, but when you're messing with large sets of data, knowing a few "update" tricks can make your job a lot easier!