The listing tags that are built in to MovableType (MTEntries, MTCategories, etc.) have limited number of sorting options. For example, while you may sort entries chronologically, it is not possible to sort them descending by date, while ascending by time. Nor is it possible to intermingle different types of listings, for example a list of comments and trackback pings.
The MTCollate plugin gives you great flexibility in sorting listings (for example, descending by date while ascending by time) and allows you to list heterogeneous data (for example, entries, comment and trackback pings together sorted by time). It even allows you to sort by non-continuous values (for example, categories could be sorted by other than name or id).