What does the bypass line from the electric fuel pump to the fuel cell do?

I believe you are asking about a fuel pump and fuel storage cell used to supply gasoline to an engine, perhaps in a car or other vehicle. If not, the answer below is wrong... The fuel pump can usually supply fuel at a much greater rate than the engine can use. In a carbureted engine, when the float bowl is full, the float and needle valve reduces fuel flow so as not to flood the engine. If the engine is idling or cruising with not much load, there is very little fuel flow to the carburetor. Most of the fuel just sits there in the line. If the temperature is high enough, like under the hood of a car sitting in traffic on a hot summer day, the stagnant fuel in the line can boil, causing vapor lock and a stalled engine. This is why we add a return line. Instead of just sitting there in the line, excess fuel flows through the bypass line back to the tank, where it mixes with the large volume of fuel in the tank, cooling it. The fuel circulating through the system never (hopefully) gets hot enough to cause problems. If the engine is fuel-injected instead, then the return line also serves another purpose. The pump sends fuel to the injector fuel rail, then the bypass line returns it to the tank. The fuel pressure regulator adjusts the amount of fuel returned through the return line to control the pressure in the fuel rail so the injectors have a constant pressure to operate from.