Generally you can fairly easily hear when a tank fires, and you should also generally know how long reload times usually are.
What the mod does is really just give a visual timer, but you can have a mental timer, especially on auto-loaders, since you can count how many shots they take.
(There are a few exceptions, since some autoloading tanks have different ammo count in their guns.. BatChats for example, can have a 6-round gun, instead of the usual 5-round top gun :) )