A definition of something is seldom complete until it is clear not only what the thing is, but also what it is not. That is true of strategy. I have read countless vision statements proclaiming an organisation's strategy to be "to be the best X", whatever X is. Leaders trot such statements out and then wonder why their staffs don't immediately know what is expected of them.
For a strategy to be effective, if must be clear what it is that the organisation, and it people, need to do, as well as what they should not do. Failing that, people will be able to do anything, which usually means continuing to pursue the pet projects they had before the strategy was launched, and you'll never be able to say "you should not be doing that as it is not aligned with the strategy".
Interestingly, a strategy that defines what you should not do can also be a source of competitive advantage. That is, if you can define a strategy which would require your competitors to stop doing an activity which is part of their existing strategy or operations, then it will be harder for them to stop doing that activity in order to challenge your strategy. That is something that Southwest Airlines did very successfully.
The reason why all this works, is that resources, most especially time, are scarce. So by defining what you will not do, you are able to conserve resources that might otherwise be wasted, and make sure they are focused on delivering your strategy.
This is equally true at an individual level. Time, and your attention that goes with it, is your most precious commodity. The ability to identify what you should not spend it on, and more specifically, to then not waste time on those activities, is perhaps the most valuable skill you can develop. And part of that is the ability to say "no". This can be surprisingly difficult for so many people. You need to be able to say "no" in such a way that does not get you labelled as a shirker or not a team-player. And that means always offering a valid reason for saying "no". All the better if that valid reason is that you first need to do other activities that are clearly more important and more aligned with the organisation's strategy.