What’s this hammer drill accessory and what does it do

I've had pretty good luck with Ryobi tools in the past, and the drill you are looking at will certainly do what you want it to.

Normally I'd recommend a cordless drill to just about anybody, but if you find that you will hardly ever use it, a corded drill might be better. No point in having a cordless drill if the battery is always dead when you need it because it sat around for 6 months without being used or charged.

Not really. I did this and regretted it.

The problem is that the rotary hammers that are worth it are very heavy and overbuilt (the larger Bosch SDS series etc.). They make fantastic rotary hammers but are too heavy for use as a drill for extended use unless you have really beefy forearms. In this scenario, you should get a 18v cordless drill for when you want to drill.

Then there are very lightweight rotary hammers (such as from Harbor Freight) where the rotary hammer action is extremely weak. These do a good job as drills but not as rotary hammers.