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Reactive intermediates — carbocations, carbanions, and radicals — gain significant stability when the unpaired electron or empty/filled orbital can delocalize across adjacent p-orbitals through resonance. An allylic carbocation spreads positive charge over two carbons via overlap with a neighboring pi bond; a benzylic radical delocalizes the unpaired electron across the aromatic ring. The more resonance contributors that can be drawn (without moving atoms), the greater the stabilization. This principle governs regioselectivity in addition, substitution, and radical reactions: intermediates form preferentially at positions that maximize delocalization.
Draw all valid resonance structures for each intermediate, using curved arrows to show electron movement. Rank the structures by stability (equivalent contributors are best; charge on more electronegative atoms is better). Compare the stability of an allylic cation with a simple secondary cation to see why allylic/benzylic positions are favored in SN1 and radical reactions.
From your work on resonance and formal charge, you know that molecules with delocalized electrons are described as hybrids of multiple resonance structures, and that the real electron distribution is a weighted blend of all contributors. This same principle becomes the dominant factor controlling the stability — and therefore the reactivity — of organic intermediates like carbocations, carbanions, and radicals.
Consider a simple secondary carbocation, like the one at carbon-2 of propane. The empty p-orbital sits on a single carbon, and the only stabilization comes from hyperconjugation with neighboring C–H bonds. Now move that positive charge to the allylic position — the carbon adjacent to a double bond. Suddenly the empty p-orbital can overlap with the adjacent pi bond, and you can draw two resonance structures: one with the positive charge on the original carbon, and one with it shifted to the carbon two positions away. The charge is spread over two carbons instead of concentrated on one. This delocalization lowers the energy of the intermediate substantially, which is why allylic carbocations form far more readily than comparably substituted non-allylic ones.
The benzylic position takes this further. A carbocation, radical, or carbanion adjacent to a benzene ring can delocalize into the aromatic pi system. For a benzylic carbocation, you can draw resonance structures placing the positive charge on the benzylic carbon and on the ortho and para positions of the ring — that is four or more contributing structures. The extensive delocalization makes benzylic intermediates remarkably stable. This is why benzylic halides undergo SN1 reactions with surprising ease, even when they are technically primary substrates: the intermediate carbocation gains enough resonance stabilization to form readily.
The practical consequence is that resonance stabilization governs regioselectivity. In electrophilic additions to conjugated dienes, the intermediate that places the positive charge at an allylic position is favored over one that does not. In radical halogenation, abstraction at the benzylic or allylic position is preferred because the resulting radical is resonance-stabilized. When you evaluate competing reaction pathways, always ask: can the intermediate delocalize its charge or unpaired electron? If one pathway produces a resonance-stabilized intermediate and another does not, the resonance-stabilized path will generally dominate, even if other factors like substitution patterns might suggest otherwise.