Very simply, it's for you to be able to be sure that you understand what I mean by any particular word or term or phrase or expression that I use.
I like to be understood rightly. When I say something, I want you to know what I mean.
It is a simple fact in today's world that very often two speakers or writers who use exactly the same English word or term do not "mean" the same thing as one another by that word or term!
This problem (disparity of definitions and meanings) underlies many "misunderstandings" and "wrong impressions" that people have of one another, and often even "hurt feelings." Picture the conversation between some people when one of them suddenly looks offended and turns and walks out, and the "speaker" is left looking perplexed, and asks, bewildered, "What did I SAY!?" He is told, "Nothing! You were being quite respectful, and even nice! I thought what you were saying was very good!" Well, sometimes the "problem" is that the use of a single word or term that means one thing in the vocabulary of the speaker (and maybe even something quite "benign"), means something quite otherwise in the thinking of his hearer.
I have found this to be the case, particularly so, in the world of "religion" or "theology" or "spiritual matters" in "communications" between Christians whose "upbringing" or "training" or "development" or "past experiences" are different one from another. Very often one Christian has a "church vocabulary" or his personal "religious vocabulary" whose definitions for words, terms, or expressions are extremely different from those of the other Christian he is speaking with. That simple fact often results in ye olde proverbial "failure to communicate." Two Christian brothers can often be found "talking past one another" and each not "getting" what the other brother even "means" by his words.
So: The purpose of the "DICTIONARY" section is to explain what I mean by certain words and terms, so that I will be understood.
My "dictionary" or "glossary" may take the shape of a couple of different "formats": You may be reading the text of a page and discover that one word in a sentence is a "hyperlink." Clicking on that one word will take you somewhere where that particular word is "defined." In some cases a word's definition may fill a whole "page" devoted just to that word. But in another instance clicking a word could take you to a page where several words are "listed" and their definitions given.