If you only use dictionaries and thesauruses for their most basic puropses, they're pretty boring. A dictionary defines words you don't know, and a thesaurus suggests words similar in meaning or connotation so you don't have to keep using the same ones. But leaving it at that is like saying Mt. Everest is pretty big, or that the sun is quite warm.
Dictionaries and thesauruses teach us about words. They show us how to use words properly, they open new realms of knowledge to us (what field of study doesn't have its own jargon?), they help us avoid saying the wrong thing and assist us in understanding what other people are saying.
A good dictionary doesn't just include every possible definition of any given word, it tells you what part of speech that word is, how it can be used in a sentence, how it has been used by past authors, how to pronounce it, and even what language the word derived from. How are all these useful? They provide context, and the less context a word has, the less meaning it has.
One reason communication seems to have deteriorated over the last several years is that literacy and the desire for a large and flexible vocabulary have been replaced by merely functional and highly truncated forms of expression. Kids no longer call their parents on the phone to tell them they'll be late, they text "b l8. @ j's. k?" and expect that to suffice. Unfortunately, because they don't understand the signficance of words themselves, most parents let that slide, and eventually communicate the same way.
Why isn't that okay? Why don't we all just limit our speech as much as possible and strain out everything "unnecessary"? The most obvious answer is that eventually language will be so abbreviated as to communicate nothing. But the deeper answer is more fundamental and more important: while we might be able to express basic needs and facts with shortened, out-of-context words, we can't communicate ideas, feelings, or real meaning that way.
Part of the reason people don't seem to mind things moving in this direction is that ideas are no longer thought to be important, at least, not as important as filling appetites. As Christians, we rebel against this concept: God has revealed Himself to us through His Word, which contains ideas and propositional statements for us to accept and understand. To understand them, we must understand language, which is also a creation and blessing of God.
Tools like the ones you'll find here help ensure we'll never be reduced to mere grunts and coughs. A word of wisdom for young thesaurus users: make sure you understand what your replacement word means, and don't just pick entries at random to mix things up. But do use these resources, and learn to love the words God has given us and to use them to communicate His glory and His majesty to everyone.