There are lots of reasons to manage research data. Ultimately though, it’s to make your research easier.
If data are properly documented and organised, you can stop yourself drowning in irrelevant stuff and find the data when you need it – for example to validate findings.
By managing your data you can also more easily share it with others to get more credit and impact.
You may also be required to explain how you will manage your data by your funder or university.