Rank Tracker lets you quickly re-run failed and incomplete tasks without updating the rest of your data all over again. You'll need the feature whenever the search process gets interrupted or if one of the search engines happens to be unresponsive.
By default, Rank Tracker keeps all of the results in its cache for 23 hours. So if you check rankings several times on the same day, only keywords that haven't been updated will be included in the search. This lets you keep ranking progress reports complete by 100% at no extra time and without any extra queries to the search engines.
The feature is on by default. If for some reason you don't want the results to be stored in cache or need to update the rankings more frequently than once a day – you may Clear Cache and/or disable the feature completely under 'Preferences > Misc. Global Settings'. This way Rank Tracker will recheck all of the keywords in real-time, each time you check rankings.