I think the principal reason those scenes were in the film were as narrative. As was mentioned elsewhere in this or a related thread, the story was primarily about the legislation and politics surrounding the passage of the 13th Amendment and, specifically, Lincoln's role in it, but the screenwriters and filmmakers were confronted with the quandary that a movie so deeply concerned with the rights of African-Americans (as we would term them today) had no really logical place for African-Americans in it (Mrs. Keckley et. al. notwithstanding, but the government was very much a whites-only club); so they had to do some hard work to interject conversation and exposition about slaves, freedmen, and free blacks into the narrative.
I actually wondered a bit about the actual chances of Gardner letting fragile glass plates out of his sight, but it did make for an interesting exchange between the characters of Lincoln and Tad. (We must not forget that this is not a documentary, but a dramatization.)