Phrase search allows you to search a database for particular words in a particular order. It is useful for names, companies, and all concepts that consists of more than one word. Surrounding a group of words with double quotes instructs most databases & search engines to only retrieve documents in which those words appear together and in the same order.
Example: ”phenotypic plasticity”, "climate change", "invasive species"
In this way you are searching for the two words together as one concept, not separately. Consequently, adding quotation marks will get you less, but more relevant results.
Another function of using quotation marks is that you are searching for the words exactly as you have written them, all "extra help" is excluded. Although for example Web of Science, LUBsearch and Scopus have a lot less "extra help" compared with for example Google Scholar, they still sometimes add things to your search in the background (for example some variant spellings and pluralform).
Since you exclude all "extra help" when you use phrase search, you need to be extra careful to use truncation within the quotation marks to be sure to always include different endings of your word or concept.
Example: "climate change*", "functional diversit*", "nocturnal migrat*"