They are required by law to hold some in reserve for emergencies and redirections.

I think an investigation was launched when a drone caused a significant number of flights to redirect from Gatwick London, to Heathrow London and some aircraft skipped the holding queue due to running low on reserves.

Because a significant number had to divert they had to divert and wait in a holding pattern.

I don't think anyone broke the rules but I think some changes were made as those who ran the lowest fuel (to save weight and cost) also ended up with priority landing.

https://www.cnbc.com/2017/11/20/london-gatwick-airport-grounds-flights-due-to-drone-intrusion.html

I don't think anyone broke the rules but changes were made at the airport to redirect further away sooner rather than using waiting patterns.

This article's video isn't loading for me which is a shame.

TLDR; they have to have some spare in case of stuff.