See, that's a common misconception. Yes when the notation calls for an E# you would play what is normally an F on your instrument, but you are in fact playing E# not F.
The reason it's an E# is you are in a sharp key that contains an F#...you can't have both an F and F# in any scale, so if the key calls for a note a half step below F#, that note is E#. Period. The same logic applies to B#.
The key of C# major contains an E# (in fact all the notes in the key are sharped, including B#) but many players will simply call the key as Db major instead. Db major does contain an F natural (flat keys only contain flatted notes, never sharped; the opposite is true for sharp keys).
When I go around the circle of fifths I usually name the keys this way, by flats rather than sharps:
C -> F -> Bb -> Eb -> Ab -> Db -> Gb -> B -> E -> A -> D -> G -> C
That avoids having to use E#, B#, etc. but you will find that in Gb major you have a Cb
That's the one major key where you're screwed either way... Whether you call it F# major or Gb major you get an oddball note:
Gb Ab Bb Cb Db Eb F
F# G# A# B C# D# E#
In far more technical words you properly explained what I was trying to say in my first paragraph, in all honesty thank you
To clarify on what I meant by "practical use", I had something like this in my mind...two guys jamming together on a song and someone goes "What's the first note you just played on bass?" (1st fret on the E string) and the guy goes "F". It would be kind of weird to answer back with "E#". That's what I meant by "practical use". As you could probably tell from my first try at explaining the reason for it in musical notation, I'm generally a practical use guy lol.
