Grant applications generally require statistical data to provide evidence of need or as justification for the grant ask. For example, a typical 'ask' in the narrative section of a grant application might look something like this:
"[Grantor Name] strongly emphasizes the use of data and evidence in policymaking and program development."
In other words, the grant reviewers expect to see lots of data, references, and evidence that justify how and why dollars should go to your project, program, or organization.
To produce good evidence, you must spend time researching and collecting data.
Grant writing professionals will tell you that usually the actual 'writing' of the grant is really only about 20% of the grant writing process.
Therefore the other 80% of the grant writing process -- the bulk of time spent dedicated to grant writing -- is time spent researching and finding the data necessary for creating a compelling, evidence-based narrative.
That data and evidence can come from a number of different sources.