shift not works when coordinate is used

Question

In tikz, if I use the the absolute coordinates of the point, the shift key works well and the object is moved by the shift, but I use the the command \coordinate to store the point, the shift key doesn't work. I wonder why this happen.

\documentclass[12pt,tikz]{standalone}
\begin{document}
    \begin{tikzpicture}
        \coordinate (a) at (0,0);
        \draw (a)--(0,1);
        \draw[xshift=2cm] (0,0)--+(0,1);% works;
        % \draw[xshift=2cm] (a)--+(0,1);% not works
    \end{tikzpicture}
\end{document}