Sometimes, you'll need to search for a phrase rather than a word. If you use quotation marks around the the phrase, you could run into problems. Using quotation marks can mean that no part of that phrase will map to an indexing term, so you may miss important articles. On the other hand, sometimes quotation marks can help your search, especially if there is no relevant indexing term for a concept.
For example, in PubMed, dietary intake doesn't map to a MeSH term; instead, dietary is searched as a keyword, & mapped to diet & intake is mapped as a separate keyword & MeSH term. This results in thousands of inappropriate results. When "dietary intake" is searched as a phrase, you get a smaller number of highly relevant results.
Quotation marks can also be used to search phrases as keywords, even when a MeSH term exists. For example, in PubMed, vegan maps to the MeSH term Diet, Vegtarian. In addition to using vegan as a keyword, you can also use "plant-based diet" as a keyword phrase.