Why search for words that contain letters?
Most word games are won in the middle of the board, where you build across and through existing tiles. A contains search finds every word that includes a specific letter or pair, so you can weave a high-scoring play through a letter that is already down. It is the natural companion to a starts-with or ends-with search.
For crosswords, the letters you are most confident about are often in the middle of an answer. Enter them here and the finder reveals the candidates that fit around them — especially powerful when combined with an exact length.