I would say for a drastic change, for instance, if your skin is very saggy, the filler will not be able to give you the outcome because that you will need plastic surgery.
That is one explanation. Another explanation is fillers, broadly speaking, are temporary. Some people want a permanent solution, to get it done once and for all, however, the upside is obviously permanent but the downside is, if you decide that you do not like it, it is a bit more difficult to reverse.
So, some of my patients actually come in to have fillers and are actually planning to have plastic surgery a few years later; so, a combination. They want to see if they are used to it before going for the plastic surgery, I have friends who want to try out the look that they thought might look good.