You'd need to look inside that washer end of that fitting to see if it also matches the caliper port.
Ideally they should have used two opposite taper to seal the joint, which will mean the washer didn't do anything.
However, if that fitting doesn't reach down far enough into the caliper to create that seal with those matching taper, then they may have opted to use the copper washer.
Can you put a picture of the fitting end and inside the caliper where it screws in ?