SQL is an integral part of many modern RDBMS like Oracle, DB2, Microsoft SQL Server, MySQL etc. Each vendor developed their own SQL syntaxes for their own products. All over the world many government, public and private agencies use different database products from different vendors and each develop system based on vendor dependant SQLs. So ANSI developed a common standard for SQL which runs in all modern vendor databases.
Work in all modern databases.
Easy to integrate different systems.
Easy to migrate from one vendor database to another.
Less maintenance cost.
Relatively complex syntax compared to vendor SQLs.