By default, TeX centers displayed material. (Displayed material is
just whatever you put between $$'s--it's not necessarily
mathematics.) Many layouts would be better served if the displayed
material was left-justified. Therefore, Eplain provides the command
\leftdisplays,
which indents displayed material by \parindent plus
\leftskip, plus \leftdisplayindent.
You can go back to centering displays with \centereddisplays.
(It is usually poor typography to have both centered and left-justified
displays in a single publication, though.)
\leftdisplays also changes the plain TeX commands that deal
with alignments inside math displays,
\displaylines,
\eqalignno,
and \leqalignno,
to produce left-justified text. You can still override this formatting
by inserting \hfill glue, as explained in The TeXbook.