Why do the definitions of \quad, etc. have \relax in them?

Plain Tex defines \quad so:

\def\quad{\hskip1em\relax}

Why is the \relax there? It's usually used as a token that can't be expanded, serving as a "do nothing" operation after macro expansion is finished. But that use seems redundant, because it follows a command, namely \hskip.

The macro is defined alongside \enskip and \qquad in plain.tex, and in Latex in ltspace.dtx.

An idle question, but I guess the reason for it will be worth knowing.

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So that \quad doesn't eat following text while assembling the argument of \hskip:

\documentclass{article}
\begin{document}