No effect, they are purely visual. Maybe that will change some day, but for now it's just for the looks.
It's a bit odd to see them there and have them do nothing. I know they could make it statically slippery, because they've already done that at Sepang. That was one of the main reasons I bought Dubai; I saw the marbles and got excited about the prospect of having to try to avoid them. If I had known this, I would've spent the money on one of the nineties F1 cars instead. Either that, or I would've spent a few more dollars for the Nordschleife. It's so long that marbles don't really have time accumulate in anything other than an endurance race.
Well there are still different grip levels depending on which part of the track you are on, on some tracks. I actually don't have a list right now which ones have those "dirt zones" and which tracks don't, but it's not tied to visible marbles. I can try to find out whether Dubai has varying grip zones on the tarmac. Like I said, it's definitely sth we'd like to explore more in the future, but we also have to be very careful to not introduce some weird, unwanted effect on all our circuits and to also not impact performance unduly.
I think there is always less grip when you go off the racing line too far. Really dirty parts are only beside the track afaik. Astroturf for example is mostly really dirty. Although it's smooth to drive over you can very often hear the dirt coming from the tyres and hitting the underbody of your car after you drove over Astroturf. This is not only for grass and/or dirt. Note that you have less grip as long as you hear the dirt hit the underbody. This is a sound effect that actually warns you about a temporary loss of grip.
This would be your best indicator (and it's also what I rely on). Listen to what your tyres are telling you and if you hear that little "ping-ping-ping", as if driving over chippings irl, exercise caution.