This was a big thing in Counter Strike 1.6 due to it giving you in-game advantages. For example, all leagues eventually started requiring you to use 32-bit graphics because people were using 16-bit graphics to be able to see through smoke grenades. If you ran the bare minimum settings, you would be able to minimize the distractions and increase the contrast between players and the level, letting you focus on winning.

The main reason pros will use lower settings for CS:S or CS:GO is because it is incredibly important that the game runs perfectly smoothly so that it reacts consistently to their input. Since everyone is on 144hz monitors now, that means they will need to stay above 144fps at all times.

Now, most CS pros who are sponsored will have good hardware and can do this on 1920x1080, but the problem is that at LAN tournaments in foreign countries, it is much less common that hardware which can do this is available. So the pro players will purposely use something like 1024x768 because they know 100% that the computer they will be given at any tournament will handle that, and changing something like resolution would take a few days to get used to and they won't get that chance.

When I played CS:S in CAL Open, I saw a lot of people putting the graphics all the way down so their vision wasn't obscured by dust in the air, or fluffy bushes, or sun glare, or particle effects.

I never played seriously enough to go as far as finding out if it made a difference. I just liked to play a pretty game.

CS:GO isn't that graphically advanced over CS:S. A lot of the more serious players will turn off props/doodads that don't have player collision.

Turning off bloom is always a good decision IMO... just looks ugly most of the time.

Former professional Counter-Strike player here, can confirm. A lot of the top players still play on 1024x768 even on widescreen monitors (some have it stretched, some have black bars on the side). While I can't explain the technical reasons behind it, from experience I can definitely say the blocky nature of low-resolution gives you does feel like it's easier to get headshots. I always performed better at low resolutions, even with a gaming PC more-than-capable of running at 1080p 200FPS+.

My understanding is that low settings used to make stealth units easer to see in SC2 (and heroes of the storm, which is on the SC2 engine). Supposedly they've patched it in newer versions, but I'm sure people who've logged a ton of hours in low modes still prefer it.

In HotS some of the ability effects show through the fog of war, and supposedly that's easier to spot on low settings still. It also reduces the business on the screen when everyone in the game is teamfighting in one area.