In theory you can also dip lower - standard pressup / pushup, your hands on the floor are a limiting factor in the pec stretch. By raising your hands off the floor, when your chest dips it can dip lower relative to your arms meaning you stretch more and push back up from a position that adds more to the exercise.

Im no fitness expert, but I would imagine they would possibly take some of the strain from your wrists.

what's the advantage of these over using... say.... your hands?

In theory you can also dip lower - standard pressup / pushup, your hands on the floor are a limiting factor in the pec stretch. By raising your hands off the floor, when your chest dips it can dip lower relative to your arms meaning you stretch more and push back up from a position that adds more to the exercise.

My floor is free. Not voting either way.

I thought it was a pair of irons at first!

I've got these, really good for getting a deeper pushup as said already due to going down further than if you just used the floor.
I was able to carry on doing push ups even though I broke my thumb also, because the weight was in the palm of my hand when I wouldn't have been able to do a press up on the floor!
I paid around this price maybe a little less from Sainsburys instore about a year ago.

It is exactly that - saves an enormous amount of strain on your wrists. A lot depends how weak your wrists are, or how heavy you might be. It's all personal choice again, but for anyone who does a lot of press ups, this will save the wrists.

can press ups build your chest up?