The global datasets ensures the global coverage of ecoinvent inventory data. The ecoinvent database also uses global datasets as the basis of approximations for production conditions in countries where no local datasets exist, i.e. a Rest-of-the-World activity in ecoinvent v3.2 (2015) and higher is generated as an exact copy of the GLO dataset. While they are supposed to represent global average data, at the moment they are often only extrapolated from a local dataset. If you don't have data on global averages for your dataset, you can also extrapolate them from your local dataset and adjust uncertainty values.
See also the official Introduction to ecoEditor and the Data Quality Guidelines.