When you're scrolling down a crosstab in Tableau Desktop, the leftmost row label(s) are pinned to the top of the screen so you can always see which section you're in. This makes a big difference when you have hundreds of detail lines for each leftmost row - it allows you to always see which section you're in.
When you publish the same cosstab to Tableau Server, this feature no longer exists. The same feature should be added to Tableau Server.
This idea is related to the following community forum pages. The first one has a good doc w/ screenshots to help explain -
Locking the "pane" names on Tableau Server:
Lost crosstab parent column pinning when published to server?: http://community.tableausoftware.com/thread/121294