Songs are made from repeating patterns of beats and melody. The time signature tells you what that pattern looks like. If you just memorise all the notes I suppose it matters less. But if you wanted to improvise like with a guitar solo or whatever that's the rhythm you can use to improvise with.

It's the rhythm equivalent of the key/chord progression for the melody. If you know the chord and when/how it changes through a song, and you know the basic rhythm building block you can just make up notes that are guaranteed to sound great with the other instruments playing.

I took piano lessons for many years and I could sight-read sheet music ... sorta. As long as the notes were within the staves, I knew exactly what key to play. It was when notes were above and below the staves that I had to stop and count how many little lines were attached to each note to figure it out. Obviously that messed up my rhythm a bit lol.

I'm a programmer and I've spent thirty years writing music composition software and I never once even thought about incorporating traditional sheet music representation. It's just so clunky and poorly designed for the job. And when you start getting into microtones it's completely hopeless.