Ever notice how projects seem to go quickly at the beginning and then take longer and longer when you get close to the end? Or the opposite, that making a few small fixes in a codebase can make everything better? That's the Pareto Principle at work. 80% of the work takes 20% of the time, and the remaining 20% of the work takes 80% of the time. Fixing 20% of the bugs eliminates 80% of the issues customers are seeing.
The Pareto principle isn't a universal content, and it's not about cause and effect. It's a post hoc observation. It's really just fitting reality to a power law distribution. You can't use it to predict which 80% takes 20% of the time, or even to say it's why things were distributed the way they were. That's not just post hoc, that's Post hoc ergo propter hoc
But even with that said, knowing that something is going to come up and take the lion's share of the resources (time, manpower, compute, etc) is something you can use along the way. Especially when you're doing something for the first time and don't have lots of experience. If you have 10 things to do in 10 days and when you start they're all unknown that's OK. However, if you get to day 5 and you've only completed 5 of the tasks, it might be time to start worrying. Sure, you've done half the tasks in half the time, but you have no idea if you've done half the work in half the time. And the Pareto principle tells you that there's a good chance that you haven't. So maybe it's time to tell someone the situation.
The other thing to be aware of, especially when you're trying to have a big impact on things, is to remember that it often applies the other way as well. 20% of your feature set is used 80% of the time. 80% of the processing time might be spent in 20% of your code. So nailing that 20% of your total feature set first or optimizing the right 20% of the code will have a much bigger impact than starting with a random work item and then randomly picking them until you run out of work.
But remember what I said at the beginning. The Pareto principle is a post hoc explanation of what happened. It's not the cause and it isn't a real predictor. It's just something that happens a lot and you need to be aware of. And 80/20 isn't a magical pair. It could be 90/10, 70/30, or it might not even add up to 100. It's just one more thing to keep in mind. 80% of the time.