The importance of looking at the provenance of records. AAGRA’s code of ethics states “ARTICLE IV : In undertaking research, members must try at all times to examine the original, rather than printed sources, and avoid, so far as possible, the misquotation of documents or the citing as authoritative of any questionable source.”
This blog post from the Ancestry Insider looks at the transcription of a record through seven generations of copying. In each generation of copying changes, and thus errors, were introduced. http://www.ancestryinsider.org/2016/08/take-time-to-understandthe-barbour.html
Some records are indexed, for example birth, death and marriage records, but you learn much more from reviewing the original certificate than merely perusing the index.
Not all records are available on-line. There are many many kilometres of records to be reviewed in various archives.