How many times were You asked “Do You have package XyZ in the repositories?“? Sometimes You know the answer and You will be able to say “Yes we do coz I am using it.“. Sometimes however You just don’t know… So You open synaptic and search for the package name… And what if there is MUCH quicker way?
apt-cache search packagename
This is also the way to find out the proper name of the package in case of installing from the CLI… without access to the synaptic ;).
If You want to make sure that You are searching amongst the most recent packages in the repository You may want to run this command before searching:
su -c "apt-get update"