Why frame paintings under glass?

Just went to a museum exhibit this weekend. Nice show, but I noticed a relatively large number of paintings were framed under glass, and I couldn’t figure out why. I realize that works on paper are usually placed under glass, but these were oil paintings (presumably on canvas, but I wasn’t paying attention to that part), and not particularly old (they date from the early 20th Century), so I would assume that age and fragility aren’t factors. Any clues?

Maybe acts like this could be a reason.

Tuesday, May 18, 1999 Picasso work slashed in Holland
The Associated Press
– AMSTERDAM, Netherlands

"It can happen so fast, not even an alert security guard can prevent it: A vandal tosses acid at a masterpiece or unsheathes a knife and carves the canvas into ribbons.

A weekend slashing attack at Amsterdam’s Stedelijk Museum of Modern Art that severely damaged a Picasso valued at up to $7.5 million stunned museum officials. The vandalism by an escaped mental patient has curators conceding little can be done to protect artworks – short of turning galleries into glass-cased fortresses."

Maybe, but there were other paintings in the show – more valuable ones, I’d even guess (bigger, older, more famous – or all three) – that were not under glass.