Here is a real world example that is happening in real time with me, and how knowing scales allows me to push through.
I am currently working on SRV and much of his material is a cover of Hendrix or Buddy Guy. Current song being Voodoo Child.
Intro, verse, chorus all fine. But if you know the song, either SRV or Hendrix version, the solo bits (scales) ramp up to a level that frankly I can't realistically ever seeing myself being able to play. A combination of lack of talent combined with age and arthritis are conspiring against me.
So, I could just give up and look for a song where my fingers don't have to fly so quickly, or I can take those solo bits, which are just pulled from a pentatonic scale and keep what I can play and fill in what I can't play with my own lick until it's time to jump back into the verse. And the band keeps playing.
Some music, like Rammstein who I was just listening to is all rhythm chords. No lead parts. No scales.
So obviously you don't need scales to play songs, knowing some chords will take you far when it comes to playing your favourite tunes. But some music, again Hendrix and SRV, the chords take you to the meat of the song which were their extraordinary licks.
There are more examples that others will add, but this is one simple example of how knowing a scale allows me to play even though it is a variant from the original.
edited
This year the diet is definitely gonna stick!
