If you've played the Fallout video games, you know that traveling through the wasteland is dangerous. Creatures and bandits are lurking everywhere, and they can turn a simple daylight stroll into a terrifying fight for your life. To get that experience in a tabletop RPG, you want rules for random wilderness encounters. I knew this when I was preparing my Fallout campaign, and I started to put together some encounter tables, but then got bored of it and abandoned them. For the first half of the campaign, I didn't make any random encounter rolls at all.
Part of my reluctance to fill out my encounter tables was that I was afraid throwing in meaningless fights would just make travel tedious. But I'd forgotten an important part of random encounters: in addition to determining who or what the PCs meet, you have to determine their motivations. If every encounter is just a bunch of enemies rushing out of the wilderness to kill the PCs, then, yeah, that would get old quickly. But what if not everyone wants to kill you? What if the people you meet are lost, or hurt, or just want to trade? What if they're looking for something? What if they're just camped out here and won't mess with you if you give them a wide berth? By adding motivation, you get more than a random combat; you get a random story.
On top of that, if you determine who notices who first, you add some strategic options. If the PCs see a roaming deathclaw from a distance, the PCs can decide to avoid the deathclaw, or maybe snipe at it from a distance, or even try to lure it into a trap if they're feeling clever.
Apart from generating on-the-spot drama, random encounters can also drain resources such as ammunition, chems, and health, which adds to the overall tension of an expedition. After a nasty encounter, you might arrive at your destination badly injured and low on bullets, which might substantially change what happens next.
I have a set of tables I like now, but I do wish I'd had them to start with.