In some situations abstracts serve as bibliographic metadata rather than a summary of the content. Examples includes cases where the content is hidden behind a paywall or, in defence, when a paper's content is classified in some way but the existence of the paper itself is not. In both cases, the abstract may help you decide whether it is worth accessing the full paper, but on its own won't give you an answer. E.g "we studied X" but not "and concluded Y".

Obviously abstracts can include a content summary as well as bibliographic metadata, but not all do.