The intron is probably to avoid potentially toxic expression in the bacterial host used to amplify the plasmids. The puromycin cassette is a selection marker that is inserted into the deletion site of your target cells through homologous recombination to allow for easier selection of deletion mutants using antibiotic selection.

Placing the puro (puromycin resistance) gene within the intron of the donor vector serves as a selectable marker for identifying successfully edited cells. The puro gene confers resistance to the antibiotic puromycin. After introducing the donor vector into target cells, you can apply puromycin selection. Only cells that have successfully integrated the donor template through HDR, along with the puro gene, will survive the selection process. This allows for the enrichment of cells with accurate gene edits and increases the likelihood of obtaining a homogeneous population of edited cells.