Cas9 is a protein with two functions: 1) locate a DNA sequence that matches the little RNA it grabs ahold of, and 2) cut that DNA at that location. The first of those functions, it's ability to locate particular and arbitrary sequences, is its comparative advantage against all other technologies we know of. The second, cutting DNA, well, works I guess, but will likely be engineered to be more useful, or turned off in order to make way for other more useful functions.

The protein is special for two different reasons: 1) it is able to locate DNA sequences with very high precision 2) and the sequence it locates can be swapped out as easily as changing the sequence of the RNA it grabs ahold of (trivial to do, can be done in a day, and can cost >$1 per target sequence to swap). Note, it is not special because it can cut DNA - there are lots of proteins that do that, and there are lots of better ways to change or alter DNA once you get to a particular sequence - but Cas9 was originally a self-defense mechanism, so it's evolutionary function in strep throat bacteria was to kill invaders by dicing up their DNA (at particular sequences that strep throat doesn't have).

Cas9 is powerful because it could be used to direct any function at a particular DNA sequence, where the sequence can be altered in the lab quickly and cheaply. As it is a protein encoded by a particular sequence, you can fuse it to other proteins with other functions to build a more powerful machine. (see [1] if you want to play with those sequences yourself.) As an experimental tool it will likely become a foundational tool used all throughout molecular biology - and for that alone is is worth it's fame. Thermophilic polymerase used in PCR is another such tool. As are restriction enzymes. As is GFP. That's the scientist's perspective.

However, Cas9 also previews the capability of directly and arbitrarily editing of a genome - a holy grail of biomedical sciences. Though unengineered Cas9 it's not great at editing a genome (we're not entirely sure why what it does even works) - but some 2-10% of the time it can actually edit a genome with fidelity. And that's good enough for many experiments (though not good enough for therapies). It has off-target cuts, and when it cuts it slices all the way through the double stranded DNA, and if it isn't properly stitched back together you have a broken chromosome. And getting payload DNA to the site that Cas9 cut is still really tricky. It's also a multi-part system (it needs it's little RNA as well as the protein itself), and so it's hard to deliver directly as a therapeutic. So the wild-type Cas9 is likely limited in its direct therapeutic relevance in terms of pure genome editing. But it will be used extensively for its ability to 1) further research quickly and cheaply, 2) prototype what genomic changes would do if they were successful (when you only need 10% efficacy to conduct a study), and 3) act as an engineering platform upon which other functions can be placed, and its own wild-type limitations can be overcome.

It's powerful. It's not perfect, there's lots more engineering to do with/to it. It's not going to get to the holy grail of genome editing all on its own, but it's a very solid platform to start building off of, as well as simply being a solid tool that will become a workhorse of further synthetic biology.

[1] https://serotiny.bio/