To relieve your lizard of stress. To halt/prevent 'nose-rub'. To stop irritating scratching noises. To stop your glass being scratched. To stop your lizard wearing it's claws away. But I suppose none of these are concerning.
It won't stay in because YOU let it out, to no benefit to the Dragon. Maybe if you tried to cover up the glass it would stay in it's vivarium and not constantly want out to sit on the sofa.
Also it ain't always as easy saying just let it out and then pop it back in. Certainly not in my case. Many of my animals that scratch at the glass and 'window surf' are wild caught. So are very skittish, extremely fast and many are smaller than an owl's penis. It's one of the reasons I'm not a fan of complete glass and plastic vivariums. Covering, as opposed to opening, the vivarium doors is a must with many nervous WC, and sometimes CB, species.
Gavin.
Why bother was in relation to this particular beardie, which I'm assuming is captive bred, I wasn't refering to your collection as I have no idea what you have, except they are mostly wild caught. Mine goes in on his own accord. Why should he be restricted to his viv if he wants to come out. Australia is a little bigger than my viv, not much I grant you, but a little, and a bit more interesting. Wearing his claws away? no more so than walking on sand rock or wood, oh and by the way, they need to wear a bit or they get deformed from being too long. No benefit to the dragon? In who's eyes? Yes wild caught will be more skittish because their not in their natural environment, so it's to be expected, and in this instance then by all means cover the glass so they can't see out, but then you can't see in, so whats the point of having them, may as well leave them where they were to enjoy a stress free life, except for worrying when the next person was going to try and catch them to be put in a dark Viv. Small lizards generally do move quickly, they have to to avoid predation, I have some dwarf day geckos in a glass viv (sorry) whilst they are small, I'm not sure how small in relation to an owls penis never having seen one, they hide only when I open the viv doors to put the food in, but as soon as they hear the latch click they are on the hunt.
As for nervous, all animals are nervous around humans, as we generally hunt/kill or capture them, or something bigger than themselves incase they get eaten. Puppies kittens, fish, mice, birds, horses the list goes on, but with more interaction they become used to you and tamer allowing more and better interaction. So yes, why bother to cover the glass with film, which will come off anyway or scratch badly every time you open the doors to feed your reptiles.
You may not like all glass or plastic vivs, and probably don't like the mesh ones either, but thousands do and their retiles I'm guessing are doing just fine because they are being cared for the best their owners can (in most cases). I would also hazard a guess that there has been a fair amount of research and development by these companies that build and sell these all glass plastic mesh vivs, before they decided to do so, or are they just interested in making money with no concern for the animals going in them or the people buying them.