It's not just handwashing after, but other things, like a certain gender that has an affinity for scratching certain places in the bathroom....
But mostly this:
From WebMD:
" Close the toilet lid when you flush. "Flushing aerosolizes all the organisms found in feces, and there are 3.2 million microbes per square inch of toilet bowl," says Duberg. That means that when you flush with the lid up, it's like pressing the nozzle on a spray canister full of infectious organisms."
So I don't leave anything in our bathrooms if I can help it. ( i.e., easy in the 3 of our bathrooms that have a door between the toilet and sink area, but the guest/powder room is one room, and I have a table with some essentials on it. I keep antibacterial wipes in there.