Thanks for your reply. I've cleared some doubts I had. Anyway, that was not exactly what I wanted to know. I'll rewrite the question: what I don't see is which is the difference between those two pseudo-coded functions.

rendrer_function()
{
invisible_scanlines_cpu_cicles;
for_each_visible_scanline
{
run_cpu_cycles;
render_scanline;
}
}

render_function()
{
switch(scanline)
case 0: do_something; break;
case 20: do_something; break;
(...)
}

Which advantages and disadvantages has each one?
Thanks again.