A Launch Amplifier is the first Amplifier in a Multi Switch installation, typically near the LNB and Dish installation. It is designed to boost your signal to get more mileage in a long cascaded installation.
Using a Line Amplifier you can compensate for cable losses further down the trunk lines of your satellite distribution system.
Note that due to the laws of physics there are limits to how many Line Amplifiers you can add into a Multi Switch cascade before your signal quality deterioates beyond practical use. You may want to check out the TMS 553 MultiTAP too.