It may be useful to give certain users access only to data from a certain date.
That may be the case when you have to share data with specific people and for security and/or privacy you don't want to share historical data with them. This way, should their account get hacked, not all data is leaked.
Also this way, should for example in the past personal data have been tracked and that is no longer the case, then sharing only the recent anonymously collected data becomes easier. If you gave someone access to all historical data which also includes personal data, then achieving this data sharing becomes a lot harder.