A proper data management plan will help you manage your data during the project and meet the requirements of your funder. It also helps you and others reuse the data.
A data management plan should specify
What kinds of data are collected or reused?
How will the data be documented and its consistency and quality be controlled and documented?
How will the data be stored and backed up and how will you control access to keep the data secure?
How will ethical issues and copyright and Intellectual Property Right (IPR) issues be managed?
How, when, where and to whom the data will be made available and how and where the data with long-term value will be made available and what are the estimated costs in time and effort to prepare the data for preservation and sharing?