Whilst good data management is fundamental for high quality research data and therefore research excellence, it is crucial for facilitating data sharing and ensuring the sustainability and accessibility of data in the long-term and therefore their re-use for future science.
The data you generate while doing research could end up being as important (or potentially even more so!) than the results you actually end up writing up for publication, so a data management plan will be in every effective research proposal or grant aplication. In any case, most grant funders require you to demonstrate that you know how you're going to manage your data before they'll fund you, and many will require you to archive that data in an open repository when your research is done. You never know what future research may end up being done years from now, using yours, perhaps combined in aggregate with other's data...