dpkg will clear the available packages file of all information when given this option. There are times when the current file has been corrupted or otherwise broken in some fashion. This can happen for a number of reasons.
The most common reason is that it has gotten updated from a ``broken'' Packages file. In extreme cases, changes to the file format have resulted in files that failed to work with the old version of dpkg. This requires that the new version be installed before the file can be fixed. Cleaning out this file allows the upgrade to go forward. Once completed the available file can again be updated with the new Packages format and the installation may progress unhindered. Executing the command:
dpkg --clear-avail
is a way to clean the slate and start over from scratch and will typically be followed by a dpkg --merge-avail command.