Actually E= Fb is different considering each in relation to the music idea of each key.
Fb would be used in the key of Cb whereas E would be in the more accessible keys such as G or C or A.

Using a capo means that you are technically physically playing in standard guitar keys
although the sound may come out in the key of the other instruments.

The capo won't help with a lot of modulations in a song.

The tessitura or the working range of a piece for a singer might be Eb to Eb
but this doesn't mean that the song has to be in Eb major, Eb minor,Gb major or any other key; the range depends on the musical context. For example if you sang a blues in the key of C major, Eb might be a high blues note or a low one, the flatted third of C. Jazz musicians have written charts like this, rarely, but they do exist.

Music is a language and is written as well as "spoken" so context is everything.