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<p>The essence of your argument is: If you don’t need to “know” it after taking a class, then why learn it at all.
The other variant of this argument is when people say, “Don’t worry about learning everything because you can always look it up later.” The point is to gain intuition so that you can build on the concepts in higher level classes. In mechanics, you may not remember everything, but when you get more abstract by applying things like momentum to electromagnetism and waves, it will be easier to learn because you have a feel for it.
However, in order to gain this intuition, you have to know how to learn it very well at one time.</p>
<p>It doesn’t matter as much if you forget some of the details. However, the details are easier and faster to refresh when you know . When you first learn an equation, you should be struggling a little bit to figure out why it makes sense rather than taking it at face value. Once you figure it out, though, the next time you need the equation you can look it up and understand how and when to use it, and why and in what situations it is appropriate to use it.</p>