Normally, any change to an expanded text ad triggers an ad review process, which both takes time and wipes out all stats for the ad. On the other hand, the ability to quickly tweak an ad is often beneficial: Small changes in the expanded text ad may have drastic impact on the perceived relevance. Compare the following ads:
"Sale ends in 14 hours" conveys more urgency than simply "Sale ends soon". Furthermore, if the prospective customer sees your advertisement again three hours later, he will see "Sale ends in 11 hours", convincing him that the clock is indeed ticking.
Parameterized ads allow you to change numerical values inside of an ad text without having to create a new ad. Furthermore, the values of the parameters are tied to keywords, so different keywords may trigger different versions of an ad.
In order to implement the above example, one would set up an hourly script that adjusts the remaining time of the sale.
See also the ad customizers feature, which allows the insertion of any text, not just numbers, into ads.