Citing data is important in order to acknowledge the work of others if the data are not your own; and even if they are your own, to enable others to find (and perhaps re-use) your data, increasing your research impact. Citing your data enables others to cite your work, contributing to metrics that document your scholarly works and contributions in your field.
Open data is data that can be freely used, re-used and redistributed by anyone - subject only, at most, to the requirement to attribute and sharealike.
What characterizes open data?
-From the Open Knowledge Foundation's Open Data Handbook