Many ordinary words have synonyms with the same or similar meaning. You might pick a very commonly used key word to search for but its synonyms will not be matched. You may thus miss finding relevant documents.

For example, if you search for 'dog' you will not find references to 'hound' or 'canine friend'. Similarly, a search for 'train' will not automatically find other foms of the same word (e.g. 'trained' or 'training'.)

However, note that plural words are automatically matched with singular words so that a search for 'dog' will also find 'dogs' (and vice versa).