To form the triangle that locates the rear axle. Without it, the seat stays (?) would flex and break.

Most bicycle frames are two triangles sharing the seat tube, making it strong and rigid.

But at least that bike is theoretically possible if it was built rigidly enough that it didn't flex too bad.

The interesting thing was that many of the drawings were not physically possible at all, like connecting the chain to the front wheels, or some other bizarre mistakes.

> But at least that bike is theoretically possible if it was built rigidly enough that it didn't flex too bad.

True, but a construction would stand a good chance of being too brittle and subject to catastrophic failure during riding involving changing loads (or bumpy roads).

Connecting the chain to the front wheel is certainly possible. For example, take a look at the Cruzbike http://www.bentrideronline.com/wordpress/wp-content/uploads/...