We use this tool specifically for emptying out the fuel filter canisters on late models TDIs, especially newer common-rail engines, that have cartridge style fuel filters.
When you remove the fuel filter on these vehicles filtered fuel cross contaminates with unfiltered fuel. Worse, when you remove the old filter some contaminants may fall off the filter back into the fuel. This means the remaining fuel in the canister is unfit to run through your engine's advanced fuel system.
This tool allows you to quickly extract the remaining fluid from the canister without going through the work to remove the canister to drain it. With a 1 liter capacity the syringe can easily empty the fuel canister.