Vesicles

Why do neurons have synapses at all? Why not just wire dendrite to axon directly?

Vesicles discretize synaptic events. The "quantal count" is the number of vesicles per synaptic event; by interposing synapses, rather than just passing the electric potential directly, the signal is rendered digital.

Presumably a more important aspect, synapses allow the trans-cellular environment to affect the signal, e.g., NO.