I first started drawing in Biro while on holiday. For want of something to do with my hands while chilling out I grabbed the nearest pen and pad. It just happened to be a biro and my notebook. Easy to use, no mess, no tidying up and you can slip them in your pocket.
Okay, the colours aren’t subtle and using them takes some getting used too but it’s part of the challenge. I really enjoy the freedom and the lack of value attached to them which means they have no sales potential because I can’t guarantee the permanence of the ink. Biro does fade if left in sunlight. Fixative or varnish will distort the colours so keeping them in a shaded room is as good as it gets.