Metadata are the data about data. They can include keywords (to facilitate search for the data and links to similar data), links to the research articles that have used the data, descriptions of variables (to improve (re)usability) and the instruments used to capture the data (to facilitate recreation of the data or creation of similar data).
You may be unable to share your data if, for example, you do not have patient consent to share, or because you have used proprietary data provided by a company. In such cases, a metadata-only record provides a venue for extensive details so other researchers can understand the considerations you made when gathering data, the tools you used, and the methodology you followed. This can be extremely valuable to researchers working in the same area to help them refine their own experimental techniques and may lead to future collaborations. Metadata can also provide details of who should be contacted to provide access to the data, if this is an option.
A further advantage of a metadata-only record when the research data are not openly available is simply that it allows readers to easily assess whether or not they want to request the data. A well-written Data Availability Statement performs the same function, but a metadata-only record in an appropriate repository has the advantages that it can be found via a data-focussed search and the data can be described in more detail via rich (and linked/interoperable) metadata. As the metadata should mention the related Article, this can result in views and citations of the Article.
Here are some examples of metadata-only records: where the data are not openly available due to participant privacy, where access to the data is restricted but the metadata are open, where metadata were created to enhance an existing dataset, where a project has been described for which the data may become available at a later date (data were not linked to this page at the time this blog was published).
In wrapping up, it is also worth noting that metadata-only records do not come without some possible disadvantages. Namely, they can lead readers of the article (and, in particular, automated bots that trawl articles looking for data) to believe data are available. To minimise this problem, ensure you clearly flag that the record is metadata-only (i.e., in the title, description and any relevant fields).