The common thought is that serif fonts are easier to read so that's why it's suggested to make the text a serif font.
Most sites though go with what's aesthetically pleasing rather than what's easy to read (as long as it doesn't actually hinder reading).
Serif fonts are usually seen as warm, traditional, or intellectual while san-serif fonts more modern and clean and uncluttered.
As to the reality of what's easier to read, it may not be as clear cut:
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