This demo explains how the tooltips can show data from the datasource without actually performing an AJAX request or calling a WebService each time one is shown which can ultimately increase the server performance.
In this demo data from the grid's datasource is concatenated for each tooltip, but the general case is to just get a string, e.g. a localized hint related to a button on the page.
This logic is suitable for data that will not change much over time and not all of it is necessary in the main view of the page. The approach is similar to the Data Caching mechanism, but avoids the tooltips' requests to the server at all (except the ones for the images, of course, but they are simple GET requests and not postbacks).