1) Ease of understanding
DFDs, ERDs and flowcharts are graphical tools, which are easier to read through and understand as compared to blocks of narrative texts. This is because they employ the use of text and diagrams which the mind can easily comprehend.
2) Easier to identify and correct errors
Since the tools are graphical, errors can be quickly spotted and corrected as compared to narrative texts which force one to read sentence sentence in order to decipher the meaning and thus identify errors.
3) Ability to summarize the whole system in a small space
Context level DFDs (The highest level of a DFD) summarize a whole system in a very small space. It‘s also possible to produce a compact (one page) system flowchart summarizing the basic system functions and processes. ERDs can also be constructed to fit a page for summary purposes.
Diagrams reduce the ambiguity that could feature in narrative texts due to ambiguous sentences and paragraphs since they reduce the amount of text involved in describing a system
5) Presence of standards, which assure quality of designs
DFDs, ERDs and flowcharts have well-defined diagramming conventions. Adherence to these standards ensures uniformity of system designs, which thus aids compatibility, and understandability of system designs.