It is necessary for both cleaning and relubrication of the parts.
It's not necessary to do it every time you clean the gun. I've got an S&W Model 19 with several thousand rounds through it without having the side plate or (or having been cleaned, for that matter!).
Word of advice about this...
1. Remove the screws, and keep them in order so that you can replace them in the same holes.
2. NEVER pry the sideplate off! The proper method is to cup the gun in one hand, side plate up, with a thumb or finger very lightly over it to keep it from bouncing off and hitting the floor, and striking the grip frame with a wood stick, rawhide mallet, etc. The sideplate will jump loose.
3. When putting the sideplate back on, make sure the little lip on the top of the side plate is properly positioned under the outer edge of the frame.
4. Install the screws in the proper holes, and tighten them as you would lug nuts. Do NOT tighten one all the way down and then move to the others! Tighten in a circular pattern, a little at a time.