Wikipedia explains it well:
Form follows function is a principle associated with modernist architecture and industrial design in the 20th century. The principle is that the shape of a building or object should be primarily based upon its intended function or purpose.
The architects who developed this concept might well have borrowed it from a spider for the shape of this web certainly meets its intended purpose — a circular pattern of silk threads designed to capture lunch.
It’s not a perfect trap. Stuff happens. Catch a bird (or a careless hiker) and the whole thing goes for naught. A major do-over is required. But flies or little bugs, that’s a different matter. They can be subdued and stored in the cupboard or eaten right away.
Unlike a building, a web can have holes and funky patches. No matter, since the web owner is architect, builder and site inspector. Whatever works, works.
Form and function in action. 🙂