It's their custom. You gotta wash yourself before entering the tub as some use the water from the tub to wash clothes or somebody might use the tub later. It's an economical use of water. I learned about it from YT and my Japanese teacher. (Korea and Japan has some similar ways)
THAT PISSED ME OFF TAKE OFF YOUR SHOES MAN AND WHAT WAS THE POINT IN TAKING A SHOWER IF YOU WERE GONNA GET IN THE BATH TUB- let me go on about my day