IMO I think it's useful because some site owners put a lot of useless junk / filler in their article when a simple answer is expected.Rarely when I see the answer on a Google snippet do I even go to the originating page, unless I think their may be more detail that I would like to know.... but generally when I am doing a search, I am looking for the answer, and if the snippet provides that, I have no need to go to the site... I'll use the answer I found and fix the problem that I was asking about and then go about the remainder of my rat killing.
For simple answers, I get it straight from the Google search page.
But sometimes I find myself having to click the result because the snippet may have answered part of my question but there's more I need to read to understand why the answer is what it is.
But as a site owner, I do notice an influx number of visitors when I get my content showcased at the top like this.
So I don't mind at the end of the day.
If you have a problem with it (generally speaking), you're free to opt out like @eva2000 pointed out.