Why should scientists provide an abstract for, or summary of their research?
Well, it's because research is specialized. You might have the most compelling results ever. Should I be interested in it?
The research literature is absolutely huge. Sometimes it is hard to put your research in the right context. Has your work been done before? Is your research relevant? What are the implications of your research? What other approaches have other researchers adopted in relation to your research question? Given the volume of research (in all areas, not just Chemistry), it is worthwhile to give a brief summary so that your research relates to the wider research area.
Your introduction will also be abstracted by the various citation searchers (such as Chemical Abstracts Services ) so that other researchers can find out what you've done, and where you published it. Quite clearly, given the volume of publications, a brief precis in the introduction is very useful.