It does add more tension, which prevents the filament from slipping as it feeds in and out and helps overcome any friction challenges between the CFS and the Printer. With less tension, friction can stop the filament from feeding all the way and the splitter motor in the CFS will continue to spin and try and push, eating away at the filament. The goal is to deliver the filament as reliably as possible to the printhead sensor so the motor will shut of properly. More tension improves the likelihood of that happening.
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Also, as far as STL for the washers, I don’t have any. I just used the disc shape in the slicer to make a spacer 1.5mm thick and 7.9mm diameter. It can be a solid disk; it does not need a hole in the middle like a traditional washer. It is pretty simple to reproduce within the slicer itself, so I never thought of saving the STL to be honest.
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Thank you very much !
It is clear for me now, I will recreate the object and try it. Thank you so much ! 
I can not stress enough how important that spring tention is. I was having issues with getting filament to the extruder on both K2+’s (each one has 2 CFS’s on them).
I tried the easy fixes first (reducing bends in PTFE tubes, shortening them, etc) with no change so I rolled the dice and did the spring mod on all 4 CFS’s by clipping the first ring off of the spring. One one machine this made little difference but I was able to finally correct it by fixing the PTFE tube right at the extruder.
The 2nd machine with the other 2 CFS’s made it worse, oh so much worse. It went from not reaching the extruder to chewing up the filament. I was able to fix this by placing washers under the springs to increase the spring tention and that resolved it.
TLDR: If the fiament is not reaching the extruder, you need to replace/rearange your PTFE tubes. If you filament is getting ground up in the CFS, you need to either clean the feeder wheels or increase the spring tention.