Why can a thesaurus make your writing worse?

Because a thesaurus lists words with related meanings, and people use it as though the entries were interchangeable. They almost never are.

What a thesaurus does not tell you:

Connotation. Slim, slender, thin, skinny, scrawny and emaciated all denote low body mass and carry completely different attitudes. A thesaurus lists them together and says nothing about which flatters and which insults.

Register. Buy, purchase and procure mean roughly the same thing at three levels of formality. Substituting the formal one into casual writing sounds pompous — the most common symptom of thesaurus overuse.

Collocation. English has strong conventions about which words go together, and they are largely arbitrary. You make a strong argument, not a powerful one; you drink strong coffee, not forceful coffee. Tea is strong; rain is heavy; a smoker is heavy. A thesaurus gives you the synonym and not the company it keeps, and violations sound distinctly non-native.

Frequency. Many listed synonyms are rare, archaic or literary. Reaching for an unusual word makes a reader stop, and stopping the reader is a cost that must be earned.

Grammatical behaviour. Near-synonyms often take different prepositions or complement structures.

Why writers reach for it anyway. Usually to avoid repetition — but repetition is far less objectionable than most people believe. The Victorian rule against using a word twice, sometimes called elegant variation, was criticised by Fowler a century ago and produces writing where a reader wonders whether the vehicle in paragraph three is the same thing as the car in paragraph one. Repeating the plain word is usually clearer.

When it genuinely helps: when you know the word you want and cannot retrieve it, and when you want to survey options before checking one properly.

Always verify an unfamiliar synonym in a dictionary with example sentences.