Relief is necessary because a string vibrates not only sideways but also away and towards the fretboard, and the oscillation center is in the middle of the free portion of the string. If completely flat, the action has to be unnecessarily high to keep, say low e from hitting frets on the middle of fretboard when plucked open. Some relief means more room to vibrate on the middle while the action stays low further up as the fretboard curves to meet the strings closer.
On best necks, there is more bass side relief than what occurs on treble side as on treble side strings vibrate much less widely.
The more heavy-handed you play, generally the more relief works better, while a gently picking shredder may like even totally straight a board and doesn't play open hard notes or mind a little fret noise.
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Dee
"When life's a biatch, be a horny dog"
Amps: Marshall JVM 410H w/ Plexi Cap mod, Choke Mod & Negative Feedback Removal mod, 4x12", Behringer GMX110, Amplitube 3/StealthPedal
Half a dozen custom built/bastardized guitars all with EMG's, mostly 85's, Ibanez Artwood acoustic & Yamaha SGR bass, Epiphone Prophecy SG, Vox Wah, Pitchblack tuner plus assorted pedals, rack gear etc. for home studio use.