Silicone washers eliminates direct contact between a 3 piece razor handle and base plate. If you are a collector of razors or are simply someone intent on keeping your razors in top condition this is certainly something to install on your razors.
From a practical point of view there are pros and cons; I find them a bit cumbersome to place and remove continuously as I disassemble, rinse and dry my razors after every shave, they are small, my eyes are old and my fingers are not the most delicate tools out there. On the pro side I have seen a few (even modern day) razors where corrosion have set in, and it is always best to never expose the bare metal beneath the coating. This is the joint where most friction and wear will take place, and the first place on your razor that will show marks with normal use. From an aesthetic point of view we all love a clean, shiny, new looking razor without marks, and if this is one of your priorities you'll make the effort to keep them in use.
It is silicone, and it is thin, although they are tough and very flexible, take care not to tighten the razor handle so much as to have them look like a fat guy with his boep bulging over his belt! I actually had to restrain myself getting it just right, erm... not my boep that is...that is still solidly in place!
After a bit of use I thought to myself this is actually a very good 'tool' to teach folks the little amount of tension that is required to set a three piece razor head so that it is ready for use, and in the process they will avoid long term (and sometimes critical) stress on the threads.
Stay pretty and avoid the bulge folks!