Very interesting idea and useful from a number of perspectives — as a snapshot of public interest at set point in time, marketing, historical archive … well, you get the idea.
From the link:
Realizing the historic value of these commentaries and first-hand accounts, Google has begun archiving every tweet in what it calls “Replay”—a search function that presents in bar-chart-form the popularity of tweets through a period in time and lists associated tweets for you to browse chronologically.
To access Replay, perform a Google search, choose “Show options…” This reveals a toolbar on the left; click “Updates.” A graph will appear denoting the popularity of that phrase or keyword at that point in time. By hovering over the graph, you can zoom in to a more specific time of day and read the tweets that were sent in that time period.