So you want to change the world.
Let's get specific here: you've decided that some organisation is abusing its power and you want to do something to change its behaviour. You've tried talking, attending stakeholder meetings, the whole nine yards. Nothing has changed; the only thing left is a protest.
So you start to organise a protest. Why? To change the organisation's policy on a specific issue that's dear to your heart.
You put up fliers, put the word out on Facebook, on twitter. You get a whole group of people together, people who agree to show up outside the organisation's offices. Why? To attend a protest.
Now, how many of those people share your specific "why"? How many of them are there with their own "why"? How many have a different beef with the company? How many just like attending protests?
Because if the majority don't share your "why", you may not have a protest any more.
You may just have organised a riot.
These kinds of compromise happen any time you have a group of people who unite behind a "common goal". Everyone is an individual, and so their reasons (their "why") will differ slightly. As a leader, it's your responsibility to ensure that your original "why" remains a clear navigation point.
This can be surprisingly difficult. Take, for example, online journalism. Your goal is to engage people with your writings. Your board wants to help you attain that goal. So does your team. This means you need an objective measure of that goal. You decide, reasonably, that engagement can be measured by how many people visit your site, and how long they stay on it.
Instantly, your team and board starts to focus on ways to increase page views and visit length. Before you know it, you're splitting articles over multiple pages and patting yourself on the back for "increasing engagement".
Your "what" has become your "why", and you didn't even notice.
The example here feels too obvious, and it is, but it's also painfully reasonable. Any time you have a group of people, you need to have some way of telling, at a high level, if that group is headed in the right direction. That means you need a map and a compass: you need some way of knowing. This is hard, because your goal, if it's interesting, is probably very hard to measure. That's why it's interesting. It's why there's a gap that you're trying to fill.
If it's hard to measure, it's hard to succeed at.
The danger is that your measure becomes your goal: that your what becomes your why. And when that happens, you're in serious trouble, for a very simple reason:
Your why tells you when to stop.
"What"s are usually infinitely improvable. Your "what" might be money in the bank, or visitors to your site, or cars in your garage, or kids in your house, or speed of your app, or any number of numbers that have no upper bound.
Your "why" is usually finite, or at least has a definite "end point" that you can reach and take stock. If you conflate the two, you're going to end up running in the same direction even when you've run out of road.
And when that happens, your "what" is going to become someone else's "why not?".