The drain plug is harder steal than the aluminum pan and so tightening the plug too much can strip the threads in the pan. $$$! The crush washer is softer copper. It deforms a bit and seals without over tightening. Fanatics use a torque wrench.
Common practice on VWs, Subarus, etc.
I used to have a spare drain plug and washer I somehow acquired and when I went to my local quick oil place (gasp!) I'd take em a nice clean set and make sure my baggie got the old set and I'd clean it for the next time.
https://www.ecstuning.com/b-genuine-porsche-parts/copper-crush-washer-priced-each/n0138083/?utm_source=eBayCommerce&utm_medium=cpc&utm_campaign=US.ShoppingFeed
Pelican
Or one of the dealers that sells online.