Explanation
To create a query-based In-Place Hold, a user requires both the Mailbox Search and Legal Hold roles to be assigned directly or via membership in a role group that has both roles assigned. To create an In-Place Hold without using a query, which places all mailbox items on hold, you must have the Legal Hold role assigned.
The Discovery Management role group is assigned both roles.
Reference:
https://docs.microsoft.com/en-us/Exchange/permissions/feature-permissions/policy-and-compliance-permissions