I think these are mostly marketing terms. What they're saying in those ads is that the machine has a pump that is able to provide 15 bar (or more on in some cases) of pressure. In practice, the pump pressure should be a bit more than the pressure you want to brew at so that you can reach and maintain that pressure when brewing.
Having a stronger pump isn't a bad thing (and it may be the standard because of other use cases for those pumps; I'm not sure) but it's not going to give you any benefits. It's a bit like having a race car in traffic, it won't hurt but you're not going to go faster.
The 9 bar figure you're referring to is commonly cited as an ideal brew pressure. For example, James Hoffmann mentions it in his video on pressure for espresso brewing. He describes it as an ideal brewing pressure because of the associated flow rate which depends on the amount of pressure.
Of course the 9 bar figure isn't set in stone, there's definitely room for experimentation, possibly by changing the pressure during the brew (assuming the equipment supports that). An article in Barista Magazine describes Scott Rao's experimentation with pressure profiling. He had good results with a long pre-infusion where pressure increases slowly, peaking at 9 bar followed by a slow drop in pressure. With a 60+ second extraction and 25% more solubles, he got positive feedback. One of the experiments was also featured in a James Hoffmann video back in 2019.
Is 15 bar or 20 bar a good thing to have, compared to 9 bar?
To answer the question directly, I don't think it hurts (or benefits) to have that high pump pressure. As a brewing temperature though, I don't think it's used much. As explained in the first video I linked, too much pressure may further compress the coffee puck in a way that may harm the flow of water.