A flare wrench is like a box wrench with one corner missing so you can slip it over a tube such as a fuel line. It allows you to grip more of a nut that you could with an open wrench.
FYI, Diesel fittings don't get as tight over time as gasoline fittings.
Fuel injectors have very precise orifices to cotrol flow and spray pattern and thus are a lot more sensitive to contaminants. Fuel lines are a lot less sensitive and can be cleaned by blowing air through them in most cases.