Executives and clients like to see timelines and total project estimates. I wrote an article* about this. The idea is that you do some rough planning, applying the principles of Agile story point estimation, and extrapolating a rough/relative timeline.
Relative timelines with non-specific dates are useful for getting an idea of the scope and possible time of a project. Helps determine what should be built first, prerequisites, and what other teams might need to be involved (and at what stage). Also useful to get the product managers or executives thinking a bit more deeply about the project and perhaps reconsidering it, or reducing scope if it looks like it will be larger or take longer than expected.
Once a project is kicked off, after a few sprints the initial plan can be updated based on the actual velocity of the team.
* https://jonpurdy.com/pm-framework/how-to-quickly-plan-and-es...