Why Fight for Palmyra?

Holding the Damascus region is harder without holding Palmyra. I have been starting to think Assad may be preparing for the Big Contraction of retreating to a rump Syria and transferring the capital there. But does a new fight for Palmyra make that speculation wrong?

The Syrian army said on Thursday it was closing in on Islamic State militants in control of Palmyra, in a major offensive to recapture the city of Roman ruins from the jihadists.

Why is Assad committing to a futile fight for a distant city--as the fight for Aleppo is--when he has few troops to spare?

Or does it mean Assad needs the city to cover a withdrawal from Damascus and points south?