Outdoor electrical installations face conditions indoor ones never see, and most of the requirements that apply to them exist for a specific reason rather than as generic caution.

Water is the obvious factor. Ground fault protection is required for outdoor receptacles because a wet environment makes the path to ground through a person far easier. This is not a formality. It is the single most effective protection in the outdoor environment.

Enclosures are rated for the conditions they will see. A weatherproof cover that seals only when nothing is plugged in is inadequate where a cord will be left connected, which is why in-use covers, the bubble style that closes over a connected plug, are required in those locations.

Cable and conduit selection depends on exposure and burial. Direct burial cable is rated for contact with earth. Ordinary indoor cable is not, and it fails within a few seasons when buried. Burial depths vary with the wiring method and whether the circuit is protected by ground fault protection, with shallower depths permitted for protected circuits precisely because the risk of a damaged buried cable is mitigated.

Temperature matters more than people expect. Conductor ratings are given at an ambient temperature, and installations in direct sun on a roof or a south wall can run far above that, requiring derating.

Ultraviolet exposure degrades many plastics. Fittings and conduit intended for outdoor use are formulated for it. Indoor rated components installed outdoors become brittle and crack.

Lighting and receptacles at grade level are also subject to physical damage from mowers, snow removal and general activity, which is why the mounting height and protection of outdoor devices is specified rather than left to preference.

Anything involving digging brings a separate and important step, which is locating existing buried utilities before breaking ground. Services in older properties were not always installed where records suggest, and the consequences of striking a gas or electrical service are severe.

Permits typically apply to new outdoor circuits, and the inspection generally happens before the trench is backfilled.

For more detail, see this resource