A data repository is an online storage location for your data that may also provide for long term preservation.
Why use a data repository?
Your data set will be easier to find online so other researchers (or you!) can re-use it.
Reduced requirements for maintenance and upkeep on your part
Gives your data a unique identifier (such as a DOI or permanent URL). This identifier is then used to cite your data set. More citations = greater research impact!
Possible long term preservation: Some data repositories are committed to providing long term preservation of the data sets deposited there. But carefully read the repository’s online documentation to determine if they provide long-term preservation.