War Supply Crates drop at set intervals in many zones added post-Battle for Azeroth, and once claimed for your faction provide various rewards helpful for PvP (including Conquest), as well as profession recipes that can be sold on the Auction House.
The WarCrateTracker add-on is useful to keep track of War Crate drops and have an idea of when the next drop in a zone will occur (only works in Dragonflight and The War Within zones). Tracking becomes more desyncronised as more crates are missed, and zones may unload when there are no players in them so crates from the current expansion are the most consistent. However, those crates are also the most contested, so Dragonflight crates are better to farm for Conquest idly, though some Dragon Isles zones have inconsistent issues that make tracking difficult (Ohn'ahran Plains crates don't appear on the map until they land, and zones outside of the mainland seem to drop less consistently, likely because they are less populated).
The below timer can also be used to work out when crate drops in a zone are likely to happen.
Entering a time or hitting "now" will save the timestamp as a cookie. The next time you open this page on this same browser, it will auto-populate. When the next crate drop is within 3 minutes, there will be an audible tone.
Crates drop every 45 minutes in each Dragonflight zone. The drop time differs by a minute or two, zone to zone, so the timer is only accurate for telling you the drop time of a single zone. Once you know the rough order/sequence of the zone's airdrops, they seem to be stable until the next server maintenance, after which they'll subtly change again.