I've used it in quite a few of my scripts, but I dont know what its used for.
It's not necessary in Lua, but you can use it optionally to end function calls or variable declarations.
i.e.
local var = "Hello"; -- same as local var = "Hello" print(var); -- same as print(var)
Scripting in Roblox is different from Coding, in coding, you would use the syntax ";" to run a code. At the end of the code you would probably put a ";" telling the computer that you finished that line and to run it.