During the installation of Oracle Grid Infrastructure 12.1.0.1 you've had the following option to choose YES/NO to install the Grid Infrastructure Management Repository (GIMR) database MGMTDB:

With Oracle Grid Infrastructure 12.1.0.2 this choice has become obsolete and the above screen does not appear anymore. The GIMR database has become mandatory. 

What gets stored in the GIMR?

• See the documentation here

See the changes in Oracle Clusterware 12.1.0.2 here:

• Automatic Installation of Grid Infrastructure Management Repository

  The Grid Infrastructure Management Repository is automatically installed with Oracle Grid Infrastructure 12c release 1 (12.1.0.2). The Grid Infrastructure Management Repository enables such features as Cluster Health Monitor, Oracle Database QoS Management, and Rapid Home Provisioning, and provides a historical metric repository that simplifies viewing of past performance and diagnosis of issues. This capability is fully integrated into Oracle Enterprise Manager Cloud Control for seamless management.

Furthermore what the doc doesn't say explicitly:

• The -MGMTDB has now become a single-tenant deployment having a CDB with one PDB
• This will allow the use of a Utility Cluster that can hold the CDB for a collection of GIMR PDBs
• When you've had already an Oracle 12.1.0.1 GIMR this database will be destroyed and recreated
• Preserving the CHM/OS data can be acchieved with OCULMON to dump it out into node view
• The data files associated with it will be created within the same disk group as OCR and VOTING
•  In a future release there may be an option offered to put in into a separate disk group
• Some important MOS Notes:
• MOS Note 1568402.1
  FAQ: 12c Grid Infrastructure Management Repository, states there's no supported procedure to enable Management Database once the GI stack is configured
• MOS Note 1589394.1
  How to Move GI Management Repository to Different Shared Storage
  (shows how to delete and recreate the MGMTDB)
• MOS Note 1631336.1
  Cannot delete Management Database (MGMTDB) in 12.1