Why do we need ACT_RU_IDENTITYLINK and what is the purpose of this table and why does ACTIVITI create it?

[activiti_]

mysql> DESCRIBE ACT_RU_IDENTITYLINK;

+---------------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------------+--------------+------+-----+---------+-------+
| ID_             | varchar(64) | NO | PRI | NULL | |
| REV_          | int(11) | YES | | NULL | |
| GROUP_ID_ | varchar(255) | YES | MUL | NULL | |
| TYPE_          | varchar(255) | YES | | NULL | |
| USER_ID_    | varchar(255) | YES | MUL | NULL | |
| TASK_ID_       | varchar(64) | YES | MUL | NULL | |
| PROC_INST_ID_ | varchar(64) | YES | MUL | NULL | |
| PROC_DEF_ID_ | varchar(64) | YES | MUL | NULL | |
+---------------+--------------+------+-----+---------+-------+

Please explain what gets stored in the table and why do we need this if we already have ACT_ID_MANAGEMENT??