They encourage precision and discourage letting one word usurp another's meaning (infer–imply, lay–lie, like–as). They dislike the indiscriminate use of two forms, especially opposed forms, for one meaning (categorically–uncategorically, couldn't care less–could care less, regardless–irregardless). They value consistency and historical continuity (preferring home in over hone in, just deserts over just desserts, and slough off over sluff off).
