When to use a relational database?

Flexibility for writing in SQL queries: With SQL being the most common database query language.
Modeling the data not modeling queries
Ability to do JOINS
Ability to do aggregations and analytics
Secondary Indexes available : You have the advantage of being able to add another index to help with quick searching.
Smaller data volumes: If you have a smaller data volume (and not big data) you can use a relational database for its simplicity.
ACID Transactions: Allows you to meet a set of properties of database transactions intended to guarantee validity even in the event of errors, power failures, and thus maintain data integrity.
Easier to change to business requirements