What are the exact uses of a metatable?

They're many uses for metatables. They can be used to keep data clean or help with DataStores.

Here is an example of use:

local ServerPlayers = {}

game.Players.PlayerAdded:Connect(function(plr)
local playerInfo = {}
table.insert(playerinfo, plr.Name)
if plr:IsInGroup(1) then
table.insert(playerinfo, "In group")
end
table.insert(ServerPlayers, playerinfo) 
end)

What this does is creates a table called ServerPlayers. Everytime a player joins the game the server creates a table called playerInfo and putting in it the player name and if the player is in the needed group it puts "In group" in the table. Once done it inserts the table into another table. This allows you to get all of the player data in one quick use instead of grabbing 3 tables and inserting something in them.