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 a MeSH term, so you may miss important articles. On the other hand, sometimes quotation marks can help your search, especially if there is no relevant MeSH term for a concept.
For example, 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.