the difference is in the logic of the command. in the first example, the height parameter is simply 10 meters. in the second example, you are both getting the height parameter of the given object, and also adding to it. if you were to type
setpos [getpos this select 0, getpos this select 1, getpos this select 2 +n]
without the brackets, flashpoint would most likely interpret this as trying to add 'n' to the command 'select 2' rather than the value that 'select 2' returns, which would cause an error. the brackets are used to separate the parts of the command, so you add 'n' to (the value in here).