There are two ways I find wandering monster tables particularly interesting.
Firstly, you can design your entire sandbox around them. This is particularly true with wilderness encounter tables which can fill in the blank parts of the map as the characters move through it. I have lots of different things on my random encounter tables apart from creatures and peoples, including structures such as villages and temples, landmarks, and general weirdness. I even use a shuffled card file of unique encounters (that started with the hex encounters in Wilderlands of High Fantasy). Draw a card and record it on the map. And then connect it to what is going on already (and influence what is happening ahead).
Secondly, and this is particularly true with dungeon adventures, they make a useful goad to player characters. In this case they often don't actually get used in the normal course of events, but when the characters do something that is likely to attract the attention of adversaries - like smash down doors or smash open chests rather than pick them - then they become exceedingly useful, even if play is normally predetermined. This is especially true when the characters decide to hole up for the night within the dungeon.