When searching for a tag (e.g., for the |:ta| command), Vim can either use a binary search or a linear search in a tags file. Binary searching makes searching for a tag a LOT faster, but a linear search will find more tags if the tags file wasn't properly sorted. Vim normally assumes that your tags files are sorted, or indicate that they are not sorted. Only when this is not the case does the 'tagbsearch' option need to be switched off.