I guess it doesn't matter IF they take them, but I'd argue that you have a better chance of taking a catch if you've got your palms facing the right way.
The point I drill into my kids is that you can't take a catch with the back of your hands, so why not give yourself the best chance of taking a catch, by having your palms facing the ball before you need to react?
Plus, if you've got weight from you upper body being supported through hands on knees (like the guy in the red cap in particular), it takes longer to move your hands, because you have to 'unload' that weight first.
That's a couple less things to do in the split second you have to react and take the catch, which can only work in your favour.
If it's good enough for keepers, it should be good enough for slips.
But maybe I'm an old fart out of touch with modern cricket......