Why are certain greek characters used predominantly for certain purposes in mathematics?

Why are certain greek characters used predominantly for certain purposes
in mathematics?

For example, $\epsilon$ and $\delta$ are used in Real Analysis for proving
limits... $\phi$, $\nu$ and $\mu$ were introduced to me in the context of
number theory... and then $\pi$ is used for purposes other than its
canonical $3.14...$ value, such as to represent the equation of a plane,
or as a function name.