Agree a large amount of grease/oil can make a mess, I've stepped in clumps of goo, mud, grass, rocks, etc myself at car washes left by prior users.
You can wipe off the gross clumps before you take it to a car wash if you want, or do it all by hand at home (I have), personal choice. Or if you take it to a car wash, when you're done move your vehicle up then go back and spray off the floor/grates if you've left a mess.
The reason it's required for car washes to have the oil/grit traps is because vehicles do leak oil/grease and they know people will wash their engines and undercarriages off at the car wash, or wash off half a ton of mud, rocks, grass, etc, if they've just been off-road or on the farm.