Through the evolution of a pay per click campaign, many keywords and ads will be removed for various reasons, including poor click through rate, poor conversion rate, etc. Of course, I hope you paused these keywords and ads rather than deleting or editing them so they retain their historical data (click through rate, conversion rate, etc). Paused keywords and ads are often forgotten and new variations are more likely added to campaigns over the course of time. Some of those ads and keywords that did not work out well in the past, however, may work out now.
Lets say at some point you found your ad copy to convert poorly. Therefore, you paused the ad and replaced it with a new one. Later down the road, you have found that a new landing page is causing your campaign to convert much better. You may have some old paused ads that are actually very good, but rather than going through your old ads, you forget about them. For this reason, it is beneficial to go through paused ads and keywords from time to time and see if they are worth resurrecting.
It may not only be the case that a new landing page gives new opportunity to old phrases and ads. Changes in the market can also have an effect on performance. So before you decide to add a bunch of new keywords and ads to your campaign, go through your paused ones and see if there may be some that are now more relevant.
