Precision Equals Safe Light
Parts of a Socket
When you look inside a screw base you see two major parts: A threaded outer area, and a small contact point or strip that will touch the bottom of the bulb. That center point is the socket's hot electrical contact point, and the larger threaded area is the neutral conductor.
Threads
The threads in a light socket are precisely crafted to hold threaded portion of the bulb base and to carry electricity. If the threading were any smaller, you couldn't get the bulb in---too large and poor contact could cause sparks or overheating.
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Points of Contact
The threads provide a large surface area for conducting electricity, but more importantly they ensure that the center points of the bulb and socket make firm contact.