What is the importance of schema in sql server?

Question

What is the importance of schema in sql server?

Where this schema help me? Is it important for security reasons?

Answer 1

Yes, the primary purpose of SQL schema was -is- to facilitate security management: define who [which principals] can access what [which database objects]. This was made particularly easier starting with SQL 2005 when the schema stopped being directly tied to the owner.

Another use of schema is to serve as a namespace, that is preventing name clashes between objects from different schemas.
The original use of this was to allow multiple [interactive, i.e. ad-hoc like] users of a given database to create their own tables or stored procedures (or other objects), without having to worry about the existence of similarly named objects possibly introduced by other users.
The Namespace-like nature of schema can also be put to use in a planned database setting, i.e. one when a single architect designs the database structure in a way which provides distinct type of access, and indeed different behaviors, for distinct user groups.