Once a project has ended, you may choose or you may be required to publish your data. Publishing data has many advantages. Besides helping other researchers, sharing data may also benefit you. Articles usually get cited more often when they link to the underlying dataset. In addition, if you publish a dataset, you will most likely get a persistent identifier (e.g. a DOI) for it. This makes your dataset easily findable and citeable, and helps you track its citation counts.
Research data policies
To stimulate data sharing and reuse, journals and funders are increasingly encouraging and even requiring researchers to share their data. But, not all journals have data-sharing policies. If they do, these can usually be found in the author guidelines. Big publishers sometimes list their policies on their website. For examples, see the data policies of PLOS One and Springer Nature.