It's generally called a windage tray and yes, it is necessary. Personally, I prefer to leave it in place when cleaning the upper and lower case halves but it can be removed for cleaning purposes if necessary - but to re-install it, you'll need to drill and tap holes to put screws back where it is partially secured by staked aluminum posts
I never remove them - a few minutes at your local high pressure car wash will clean that baby out - stick the nozzle right in there as deep as you can.
You'll get wet for sure.
Be sure to secure the part somehow, otherwise it will shoot across the floor.........
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