Ah - good question, simple answer....
It's because your voting power recharges at a rate of around 20% per day, so 80% will recharge back up to 100% after 1 day, if you depleted your entire VP it would take 5 days for it to recharge.
It costs 2% of your VP per full (100% weighted vote) - so you can make 10 full upvotes per day (or 20 votes weighted at 50%).
You reward and earn less the lower your VP percentage - so keeping it above 80% is better for your curation returns.
Ah, makes perfect sense now! Thank you very much!