Small mistakes can result in your tubular structures flinging chunks off into outer space. The whole script is resolved into compact subroutines, each of which represents an element of the model, and each of which always leaves the cursor where it was at the start of the subroutine. Each subroutine is therefore a small self-contained 'object'. It is possible to invest your models with some decision making ability of their own, actually considerably less intelligent than a flea, but independent nonetheless. Models can use random number generation to make themselves different sizes, shapes or colours which enables you to place a number of trees together, fill a car park with cars, fill a shopping mall with human figures, or place books on a shelf and have them look authentically variegated.
Models can work out where they are in the main project and can act accordingly.
For example, your user can specify that shadows, ironmongery or other details of trim should not be depicted at distances exceeding 50 metres from the camera. Computer models always seem to have a flat look if they lack detail, but take too long to build and render if they do have detail. It can be far faster to map a scanned photograph of something complicated onto a surface than to build the same thing geometrically. It also looks more authentic. To put scans of well known paintings into your model of an art gallery only requires one well written line of GDL code.