A thesaurus is an indispensable aid for writers - sometimes the right word is just on the tip of the tongue (or, more to the case, perhaps the tip of the finger), but refuses to come forward. Sometimes one has high praise for something, but doesn't want to use the word 'super' over and over again.
Roget's thesaurus has multiple styles of entries - main entries highlighted from the text, subentries that are very close relatives of the main entries, secondary entries that lead back to main entries cross-referenced, and variant spelling forms of words. For the main entries, there is a definition of dictionary variety before the synonyms are presented. Sometimes words have multiple meanings, and the synonym for one meaning might be inappropriate for another meaning, so the main entries break down these multiple pieces for ease of use.
Primary entries have definitions, usage examples, and synonyms; secondary entries lack the examples, and cross-reference to major entries. Homographs (words spelled the same way with different meanings) are also split into multiple entries based on this variation of meaning.
Roget's Thesaurus also uses standard dictionary labeling, so that one can identify the part of speech (noun, verb, etc.), as well as other identifying information (slang terms, informal, regional, etc.). Variations are very interesting to discover, as different words have meanings that go beyond their standard usage.
A thesaurus is a very valuable tool for those who wish to increase their vocabulary, as well as increase the richness of their spoken and written language in actual practice - it is not uncommon for one to know the words listed, but to have the presence of mind to use alternative words is another matter. Dipping into a thesaurus on an occasional basis yields rewards; plunging in on a regular basis will really enhance the command of the language.
There are few sources as adequate to the task as Roget's.