Why are allylic and benzylic radicals always more stable?
Benzylic and allylic radicals are more stable because they are stabilized by resonance.
Resonance involves a delocalization of electron density, and any process that delocalize electrons lowers the energy of a system.
We can write two equivalent resonance structures for the allyl radical, so its energy is lower than that of, say, a propyl radical.
The resonance energy of the allyl radical is about 45 kJ/mol.
The benzyl radical has even more resonance structures.
Its resonance energy is about 100 kJ/mol.