To save yourself time and frustration later! Truly, this is your opportunity to quickly evaluate a source for key ideas, important data, and helpful quotes which can easily be recalled for use throughout the writing process. Why struggle to remember where you saw a key piece of information, when a quick, organized note or highlight could easily save you the trouble. Doing this work at the beginning of exploring sources will save you some real headaches later.
Both are helpful, but the difference is perspective... Remember, an annotation is a critical evaluation (the notes) you make of a source and its key elements, conclusions and objectives. In contrast, an abstract is a descriptive summary (purpose, methods, and general conclusions) of a source written from the perspective of the source's author(s) and is typically found at the beginning of an article, essay or book.
Article: How to Critically Analyze a Source - Cornell University
Video: How to Create an Annotated Bibliography - Brock University
APA Style (Publication Manual of the American Psychological Association, 6th edition, 2010) for the journal citation:
Source: Columbia College (BC), LibGuides (Annotated Bibliography) - https://columbiacollege-ca.libguides.com/apa/annot_bib
MLA Style (MLA Handbook, 8th edition, 2016) for the journal citation:
Source: University of Nevada (Reno), LibGuides (Annotated Bibliography) - https://guides.library.unr.edu/mlacitation/annotatedbib