135mm spacing is 146mm spindle ( its not an axle...sorry engineers pedantry.....bottom brackets have an axle)....to give you 5.5mm each side to hang on the frame.....if its quick release.......solid much more.
In practice you can get away with less, as its the clamping force that holds the wheel...( think about if you tilt the wheel in the frame to get it dead central..its no longer resting at the top of the drop out)....... however, it makes it much harder to fit and removes the "saftey blanket" should your qr not be as tight as it should be or the never actually ever happened• event of a quick release being knocked undone by a leaf!
• to me or anybody i know....except the bloke who knew a bloke down the pub...it happened to his brother last week and he was killed, but us ok now and back at work.