Notification replay is mostly required to sync NETCONF clients with servers without the need to do a full RPC.
In some systems, the configuration + operational data becomes quite large, so the notification replay allows for a delta sync.
The main problem with this is that the server will only keep a limited time window of notifications; so if a client asks for a replay of the last week, the server may be only able to provide a subset of that.
RESTCONF has a different solution for this same issue, the last-modified header field(https://www.rfc-editor.org/rfc/rfc8040#section-3.4.1.1), which allows for doing a get of 'data that has changed since timestamp x'.