
You don't *need* to, but it's desirable.  HST, like all satellites in
low Earth orbit, is gradually losing altitude due to air drag.  It was
deployed in the highest orbit the shuttle could reach, for that reason.
It needs occasional reboosting or it will eventually reenter.  (It has
no propulsion system of its own.)  This is an excellent opportunity,
given that there may not be another visit for several years.