why we can specify table names enclosing them with []

Question

Is there a reason we can specify a tablename with [schema].[tableName] instead of just schema.tablename?

is it JUST to allow spaces in a tableName?

Answer 1

They're called Delimited Identifiers, and their primary purpose is to allow for special characters and keywords to be used in/as table names

For example, some genius decided the name of the primary table in a database I work with would be master, which is a keyword.

They're really not needed at all except in those cases.

Answer 2

They escape names that are not "friendly" - they can be useful if your database names contain special characters (such as spaces, dots or dashes) or represent SQL keywords (such as USE or DATABASE :-)). Without the square brackets, a query like this will fail:

SELECT column FROM database.dbo.table;

However if you write the query this way, the keywords are ignored:

SELECT [column] FROM [database].dbo.[table];

When building scripts or writing solutions that generate scripts from metadata (e.g. generating a script of all tables or all indexes), I always wrap entity names in square brackets so that they will work regardless of what kind of wonky names have been implemented (I am not always doing this for systems I control). You can do this easily using QUOTENAME function, e.g.:

SELECT QUOTENAME('table'), QUOTENAME('column'), QUOTENAME('database');

Results:

[table] [column] [database]

If you search my answers here and on Stack Overflow for QUOTENAME you will find that I use it in a lot of scripts where I'm helping folks automate script generation or building dynamic SQL. Without the square brackets, a lot of people would run into issues running the scripts.