Everyone knows that most Team Fortress 2 Leagues lack up to date whitelist files most of the times, so your newest hat, jacket, weapon reskin or taunt can't be equipped on the day Valve releases an update with new items.
whitelist.tf will respect the item and weapon bans set by the league whitelists (if you pick a preset from one of the leagues) and allows you to modify this with a couple of clicks to allow new weapons to play-test them in lobbies/mixes/scrims while also updating all the allowed/blocked Cosmetic items. Reskins will automatically be allowed if the original version of the weapon is.
If your server is running TFTrue you can use the following console variable to change the whitelist on the fly (works for public servers too if TFTrue is installed!)
tftrue_whitelist_id <ID>
Running TFTrue on your server allows for instant switching of your whitelists, without needing FTP access to upload the newest version of any whitelist file generated on whitelist.tf and is always up to date with the newest version of the Valve itemscheme whilst respecting original weapon/item bans.
Example
ETF2L S18 6v6 whitelist with ID 866 => allow B.A.S.E. Jumper & Back Scatter => Generate => ID 1150
To update the ETF2L whitelist from the start of the season to the current version with all allowed taunts you end up with 1155.
Use this ID for the TFTrue convar and play-test with the new weapons or link the whitelist to the people on your server/friends list to show which items and weapons are allowed or banned on your server.
(Allowing all new weapons and taunts on the S18 whitelists resulted in ID 1152)