They're useful to decide whether you should read this paper or another one, but they're often not useful to get a summary of what exactly the paper actually achieves. Often the abstract will imply a more interesting result by leaving out key aspects and limitations (which are detailed in the paper and its conclusions) that significantly change the impact of the paper, the abstract often is more like an advertisement for the paper than an effective summary. I mean, it may be, but if I'd read just the abstract and go away thinking, "oh, so now there's a way to do X", I'd often be wrong.
I recently read a paper whose abstract seemed to imply to me that its content was much more technical and specific than it actually turned out to be, which was disappointing. It was more useful in telling you the particular area of research than summarising its content.
They're useful to decide whether you should read this paper or another one, but they're often not useful to get a summary of what exactly the paper actually achieves. Often the abstract will imply a more interesting result by leaving out key aspects and limitations (which are detailed in the paper and its conclusions) that significantly change the impact of the paper, the abstract often is more like an advertisement for the paper than an effective summary. I mean, it may be, but if I'd read just the abstract and go away thinking, "oh, so now there's a way to do X", I'd often be wrong.