Filter-driers do exactly what the name implies.  They filter contaminates out of the refrigerant stream and and remove any residual moisture which may have been introduced during the installation and/or charging process.  They also act to neutralize acid which forms as a result of the interaction of moisture and oil.  While it has always been a good idea to use a high-quality filter-drier, the recent change to new refrigerants and oils make it more important than ever.  Filter-driers should be changed whenever the refrigerant circuit needs to be broken during service and annually on any system which uses rubber refrigerant hose.