If you own one of my pictures, you'll see that I have glass over the top of the painting, and the reverse has a backing board covering the canvas, so that the painting is presented as a 'sealed unit'.
Now, I have a friend who is a painting conservator, and he says that most of his work comes from paintings which have been ripped or damaged by the edges of chairs or tables. Hence a backing board on a painting is a no-brainer.
However, the argument I often hear is that 'oils shouldn't be covered by glass', as you can't see the texture of the paint. I would say that if you go into any major art gallery or museum, you'll see that all the paintings have glass over them.
The reason for this is conservation. Glass provides the best environment for the long-term preservation of the painting, and protection of both the canvas and paint surfaces.
Pigments can be faded and the paint made brittle by the action of UV light upon the painting. Ordinary glass cuts down UV light by around 40%. The more expensive museum quality glass is 2-3mm thick glass with a UV coating, sandwiched between two layers of anti-glare optical coating, and cuts down 98-99% of harmful UV light.
Glass also protects the textured paint surface from dust and dirt. Oil paint never fully 'cures' or dries. If it did, it would come away from the canvas.
I was told a story about a conservator who, many years ago (before Van Gogh was so famous) took a Van Gogh into his lecture to show his students. Although the painting was at that point about 70 years old, when he stuck a pin into a textured part of the paint, it came out with wet paint residue on it. (You wouldn't be allowed to do that nowadays with a priceless masterpiece, but obviously art lectures in the 60s were much more relaxed, cavalier affairs...)
I take a huge amount of care and put a great deal of thought into all my work, and I want it to leave my studio and go out into the world in absolutely the best condition that it can. Painters are always aware that their work will outlive them as their legacy, so my paintings are quality products made with quality materials, to the best of my ability. Therefore I choose the best bespoke Belgian linen canvasses, and bespoke hand-crafted, hand-gessoed and gilded frames, with glazing and backing.
Hopefully, my paintings will last for hundreds of years, give a lot of pleasure over generations, and still look as good as the day I painted them.