You can use diamonds to hurry up troop practising

https://tiny-wiki.win/index.php/King_of_Avalon_Diamond_Pack_Price_%26_Value_Guide:_500%E2%80%93a_hundred,000_Diamonds_Compared

You can use diamonds to speed up troop instruction, so you’ll have a more desirable navy to be had in much less time for battles or safeguard.