My parents were teachers and sometimes used their preps to catch their breath just like other professions have breaks, one prep to prep for potentially 6 different classes doesn't really suffice so they ended up prepping at home where it was comfortable and quiet. Prep periods are kind of a joke at this point.
Yeah if it's grading or something similar depending on the person, that often is done better in bulk/after school or whenever with lots of folks.
The prep often is maybe cleaning up or taking care of behavior stuff, getting materials organized, touching base/meeting quickly with other staff about what's working/not working, etc. Maybe writing a parent back to explain again that whatever the expectations are at home, being in class is expected while at school.
That's all a part of the teaching world but it's not like a teacher's just going to sit down and grade 40 tests on a whim with another group coming in soon.