To Organize your Research
Managing your data increases efficiency, and makes it easier to understand the details and procedures relating to your data and its collection throughout the lifecycle of the project. Sure, you remember every step of how your data was collected and how it's been manipulated today, but will you remember months from now when you are consumed by the next project? Can others follow the notes you've made to repeat your experiment? Or to re-use your data for another purpose?
To Satisify Funding Requirements
A growing number of agencies that fund research (like NSF and NIH) are requiring the data to be preserved and shared. A data management plan will help you meet these requirements by addressing documentation and preservation issues.
To Preserve your Data
The data you collected is the foundation of your research. Preserving your data ensures that your work can support future research, facilitates new discoveries, and allows other researchers to recognize your contribution. A good preservation plan also makes disaster recovery possible.
Video from Coursera's The University of North Carolina at Chapel Hill Research Data Management and Sharing, https://www.coursera.org/learn/data-management/lecture/njyKm/why-is-data-management-important