Abstracts often are misleading.

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.

I also find that briefly paraphrasing the abstract helps me understand my own expectations of the paper. Even if the paraphrase is wrong, it's still a good primer for switching into active engagement instead of passive absorption. Also, when you finish the paper you can review and correct your initial impressions; sort of like tutoring yourself (which personally I find helpful from both perspectives, tutor and tutee).