Is it to prevent the cog and chain getting caught in the spokes ?

It's to stop the chain from falling into the spokes if your RD endpoints are not adjusted correctly. If they are adjusted correctly, you'll never need it.

There is a previous, nuanced discussion here: I set my rear wheel on fire

Unless your rear derailleur gets bumped or otherwise pushed inwards, such as during transport or while parked. Having it might save you from having to replace some spokes. People derisively call them "dork discs"; I use them because they could save me from a hassle and because I don't care what other folks think. I also use a Drop Stop up front; nothing ruins a nice ride like digging a greasy chain out of a tight spot.

Don't have a greasy chain and half the problem is solved. The "derailleur pushed in" would only be an issue if the hanger is bent in the process. I've yet to have a bike or wheelset even come equipped with a spoke protector.

Of my two kids who ride bikes regularly, one absolutely doesn't require a spoke protector, and the other probably would greatly benefit from it. The first one (who is younger) is a better mechanic than I am, and the second one can barely turn a screwdriver.

People's mechanical aptitude and awareness differ. The nice thing about dork discs is they are just hard enough to take off that those who might benefit from them will likely leave them in place, or never even notice them in the first place.

Insurance for just in case if for any reason the chain comes off between the largest cogs and the spokes. Like an insurance policy, you hope you never need it but if you do you're really glad you have it. A chain coming off and jamming between the biggest cog and the spokes can really chew up the spokes and then the spokes might break a bit further down the road.

Cheers

It is a piece of plastic designed to placate riders into thinking maintenance doesn't really matter this thing will protect me. If you adjust your derailleur properly the chain cannot go into the spokes unless the hanger gets bent or you have other major mechanical failures then it doesn't matter what you have there. Take care of your bike and it will take care of you.

If your chain is exceptionally dirty, you might consider cleaning your chain more often, using a wax lubricant (or wax dipping) and maybe bringing a pair of gloves with you on the ride. A pair of Nitrile gloves don't take up much space and don't weigh hardly anything. However as they say an ounce of prevention is worth a pound of cure.

I have seen them cause plenty of issues. The freehub won't spin and instead of taking off the cassette removing the plastic disc and putting the cassette back on and solving the problem, they sit their baffled and claim warranty or just leave it for someone else. I have solved two issues of that at the shop and I am not the mechanic I mostly focus on sales and management and many other tasks.

So remember kids, tune and maintain your bike and the only plastic discs you will need to worry about are records and frisbees (or compact discs for the younger viewers)

I prefer "Rookie Ring"