Addressing this in your game does a number of things. It makes your world feel more real and interconnected. It makes your PCs actions feel like they have more significance. And - perhaps most importantly - it gives more avenues into the quest line than you might otherwise think you have or need.
An Interconnected World
The first part I want to talk about is the idea of an interconnected world in your table. Having aspects of Plot A affect NPCs and situations in Area B makes the world feel interconnected, which in turn makes the world feel more real and have more depth. Consider the real world. When something bad happens, the area around where the bad thing happened not only knows but it changes.
If bank robbers pulled off a daring heist you'll see more cops in the area. If there were fatalities the paper will have word of funerals. The news will talk about the violence. You may see people in mourning. Most likely where the violence occurred is covered in flowers and memorials for the victims.
This happens because nothing happens in a vacuum in our world. There is always collateral.
Who Is Impacted By Your Plot?
Let's return to the bandit problem I mentioned in the opening. You have bandits attacking merchants on a trade route out of the city. So who is impacted by that? The first two answers are obvious:
- The merchants who get robbed
- The bandits who are doing the robbing
But now go further than that. Who else? Are the merchants carrying vital goods or luxury goods? Both will have impact on the area not getting those goods. Food becomes scarce and people start to starve. The cost of luxury goods (or food) sky rockets. Both are likely to lead to increases in crime and resentment from the have-nots to the haves.
What about the guards? They likely have friends and family in one stop or another on the merchants route. Any guard that dies has family and friends that will mourn them. Guards that survive could be injured costing them their job, putting strain on said family and friends. Friends could go out to get revenge. Friends could attack the merchant for that revenge.
Merchants being attacked are likely going to be hiring more guards. They will put pressure on the local government for more security. That in turn will cause crack downs on other aspects of local crime making things harder. People will be punished as examples.
And all of this is without talking about the Bandits who now have supplies, cash, and notoriety. What are they doing with that? Are none of them going out and splurging? Are they just hoarding? Is it going to someone or something? Is there one town that seems remarkably resistant to the scarcity of goods caused by the bandits? Or has other merchants coming in to sell things?
Multiple Ways In
Literally everything I mentioned in the above section is a way in to the adventure for your PCs. Just like 2 minutes of brainstorming on what impact bandits would have on an area, and there are over a dozen ways for the PCs to get involved. They could come across a grieving family wanting someone to bring justice. They could notice merchants hiring more guards. They could be approached to be those guards - they look tough after all.
The More Impact, The More Reward
Finally all those ways in are also various problems the PCs solve by solving the core bandit problem. This lets you spread out reward ('actual' reward like coin and goods) across multiple grateful people as opposed to just one person. it also spreads out the feeling of making a difference. By stopping the bandits the PCs didn't just fulfill a contract for the local baron, they also avenged the friend/family member that was killed. They eased the increasing cost of goods on communities they haven't visited. They made the road safer for other travelers. And all the people involved in that can be thankful or not.
But thinking about the consequences the plot has on the world, and then how those are fixed or adjusted with the plots resolution, can turn "Alright, we killed the bandits" into "Yay! We saved the county!" And when you're trying to get your PCs to be heroes, that can make all the difference in the world.