The front-loading washing machine at my home has a window but the top-loading machine at my in-laws doesn’t have any see-through panel.
What’s the advantage of the see-through panel in front-loading machines? I did a quick search and there are a lot of theories on it.
Some say it’s to make sure that you don’t open the door while it’s operational and filled with water (just stopped due to a power cut). This could justify why most top-loading machines don’t have a see-through pane. But again most machines have a mechanical lock on door so it’s an over-engineered solution.
Another theory says that it’s relaxing for people to see the clothes being washed due to the unpredictable movement. This is why you rarely see window in dishwashers. It’s boring. This reason does sound good, but again it’s full of opinion and I wonder if manufacturers 100 years back would go with windows just for this reason.
The third reason I could find was to halt the operation if you mistakenly put your pet or child (really?) in the machine and can stop it.
Digging more, I found the first patent that introduced this concept of a see-through window. It’s by Louis Zimarik and filed in 1935. The basic idea was simple, it was a debug window. To see what’s happening when the machine is operational.
The original idea was simply an innovative debug mechanism (print statement in case you are a coder), but I guess it stayed because it is relaxing to see the random movement of clothes.