I thought I saw somewhere the idea that if a person doesn't have a grasp of key signatures, it's a sign that they simply haven't read enough music yet. I can't find the post. So here's an example of what can go wrong, from my own story, when you don't have the right guidance, or guiding material. Keep in mind that I started learning way before the Internet, and had no teacher; only two handed down books of repertoire.
We had sung movable Do solfege scales and little exercises like: do re mi, do re mi fa, re mi fa so ... patterns. So I had the sound of major and natural minor scales in my ear. I was given a little organ at 8; a starter booklet for adults of about 12 pages 90% in the key of C. I learned which piano key corresponded to the 4th space up (i.e. C). I knew "the last sharp is Ti / the last flat is Fa". So if I find Ti, I know the note above it is Do, and then I start singing what's on the page with that major or minor scale in my ear (The minor Tonic was La). Then find the Do note on the piano, play what I could sing, and try to make it sound right. That's what I had. Only that. The music I had to play was basically all diatonic, so it worked.
So if this is all you know, say your piece is in E major, and it starts descending from E. You want to hear Do Ti La So. But when you play D, it sounds off, so you adjust it to sound right (D#), ditto for C, then the B sounds right, etc. So you're feeling your way around for a whole piece - it's tedious. Since this is all I knew, mostly I stayed with one sharp / flat, or less. There was no piano after age 18, until my mid-fifties. One day I discovered, when my child was told while I attended the lesson, the repeated pattern in keys along the circle of fifths, and that door was unlocked. Sort of.
My first actual lessons were on violin. That by ear Do Re Mi works beautifully there. First levels of music tend to be diatonic. You don't really need to know key signatures: that is, my old "find Do" and then "play the sound" worked well. I learned to play scales in all the keys for major and the three parallel minors. It's simply Do Re Mi - Tone Tone Semitone - at a different starting pitch for Do. I managed to do all that without ever really understanding how to apply or use key signatures. It took three years to figure out that I wasn't reading music in any conventional sense. For violin, I was indeed playing pieces in D, A, E major, and the broken chords; some Db etc. But the sharps and flats were still no more than "Find Do through the last one".
This doesn't work on piano, because you have white keys that become black. You actually have to get a handle on key signatures; and scales. My big challenge has been to learn to relate notes on the page to piano keys, and see them - because I hear the whole melody as I stare at the notation, hear it in solfege, and all else vanishes. That's no good when music gets complicated, or non-diatonic.
It's always best if you can get a well rounded introduction to music so that you start off in the right way, and can build from there. If you did get that, it may seem incomprehensible when people get lost in "obvious" things.
Topic: Key signatures doubt