The family selection is available in two forms: in the deprecated (X)HTML ... element with its face attribute, and in the CSS font-family property.
This may be for personal taste reasons, but may also be because of some physical limitation of the user, such as the need for a larger font size or the avoidance of certain colors.
[1] A default installation of Firefox on Microsoft Windows, for instance, will display both serif as Times New Roman, and both sans-serif as Arial.
Usually only a small subset of them, chiefly the Serif, Sans-serif, Monospace and Cursive[2] are easily configurable by user in web browsers.
Many systems also perform special-cased font substitution to replace missing "well-known" families with metric-compatible alternatives.