To be honest, the answer here is nothing. SQL Server, like any other RDBMS, couldn’t care less whether your data model follows any of the normal forms. You could create one table and store all of your data in one table or you can create a lot of little, unrelated tables to store your data. SQL Server will support whatever you decide to do. The only limiting factor you might face is the maximum number of columns SQL Server supports for a table.
SQL Server does not force or enforce any rules that require you to create a database in any of the normal forms. You are able to mix and match any of the rules you need, but it is a good idea to try to normalize your database as much as possible when you are designing it. People tend to spend a lot of time up front creating a normalized data model, but as soon as new columns or tables need to be added, they forget about the initial effort that was devoted to creating a nice clean model.
Advantages of normalization
1. Smaller database: By eliminating duplicate data, you will be able to reduce the overall size of the database.
2. Better performance:
a. Narrow tables: Having more fine-tuned tables allows your tables to have less columns and allows you to fit more records per data page.
b. Fewer indexes per table mean faster maintenance tasks such as index rebuilds.
c. Only join tables that you need.