Ana, when you are in a flare, you have inflamation and Steroids help to calm that and thus, sometimes, shorten the flare...but...Steroids are not a cure of any kind for your MS.
It stopped working for me after about the third time, so I avoid steroids now.
Be sure you check for a UTI before starting steroids. A UTI can emulate a flare and steroids will make a UTI worse.