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I am not sure if there's such an expression. Sometimes, people do a lot of things in advance and it turns out that 90% of what they decided to do will never have any use to them, and there are some efficient people who only do what's necessary. Is there a word for the philosophy or manner of behaving of the people in the latter example?

How about the term "just-in-time?" It is used in the context of manufacturing. For example, it describes a factory manufacturing system where the raw materials are brought in as they are needed.

https://en.wiktionary.org/w/index.php?title=just-in-time&oldid=45591851

Example: "Acme Widget Manufacturing uses a just-in-time inventory system in their factories."

Perhaps you could adapt it to your own needs.

Both parties have planned their time in advance, the only difference is that the first party has planned everything whether it is important and necessary to them or not, while the second party has planned only the substantial and needed things. This all leads me to think of the verb "prioritize" which means:

To decide which of a group of things are the most important so that you can deal with them first.

Cambridge Dictionary

It is an expression that is very common to use when dealing with time. So, in order to use your time effectively as much as possible, you should prioritize your work.

• Prioritizing has never been a last minute thing to do; thus it has implicitly the indication and meaning of planning your time beforehand productively.

Here are three related Extreme Programming practices. They avoid doing unnecessary work now, so that you can have a useful thing both now and later:

• "You ain't gonna need it", or
  
  "You aren't gonna need it."


• If you do need it,
  Do the simplest thing that could possibly work.


• The three rules of optimization:
  
  
  1. Don't.
  
  
  2. Not yet.
  
  
  3. Use a profiler.
  
  

"Yagni" is an acronym for the first concept, which means:

"Always implement things when you actually need them,

never when you just foresee that you need them."

Even if you're totally, totally, totally sure that you'll need a feature later on, don't implement it now. Usually, it'll turn out either

a) you don't need it after all, or

b) what you actually need is quite different from what you foresaw needing earlier.

This doesn't mean you should avoid building flexibility into your code. It means you shouldn't over-engineer something based on what you think you might need later on.
This also follows the KISS principle:

Keep it simple, stupid!

The page goes on to put this practice in the context of Extreme Programming's other practices. It quotes Kent Beck as writing,

"You aren't gonna need it" is not the same as forgetting experience, acting stupid, or abandoning morals. It is a challenge to developers to abandon their fears of a far-flung future that may never happen and do an absolutely stellar job of solving today's problems today.