Back in the past, the progress towards your magic level was counted in chunks of 20 mana points. This meant that it was only optimal to use spells that are a multiple of 20, so 20, 40, 60, 80... and so on.
To illustrate:
- If you cast a spell that required 20 mana points, you would receive 20 mana points progress towards your magic level
- If you cast a spell that required 30 mana points, you would still only receive 20 mana points progress towards your magic level
- If you cast a spell that required 40 mana points, you receive 40 mana points progress towards your magic level
The light spell cost was exactly 20 mana points and that's why it was most commonly used as an optimal spell to train magic level progress.
Nowadays, this is no longer a case! There are no spells that contribute better to your magic level progress or are more optimal than the others. No matter if you cast instant spells, make runes or use wands/rods, you are getting exactly as many mana points progress as you spend (not in chunks of 20).