We use start and end dates for epics to figure out when work will be done on these epics. However it could be the case that multiple developers work on these epics at the same time, OR that not all time in this period is spent on the epic. It's very annoying that estimates get filled in when changing start and end dates, since we want to use feature based or manually set estimates exclusively.
We recently introduced a new capacity planning setting to configure how you want dates to update initial estimates. Workspace owners can configure this in Settings > Workspace > Capacity planning > Individual planning > Set initial estimates.
Selecting "Do not set initial estimates automatically" will allow you to update start / end dates on the gantt without impacting the initial estimate.