Any digital single lens reflex camera I purchase must have a built-in dust removal system.
Sensors on DSLRs must be cleaned. For folks like me, the thought of fussing with the camera sensor is intimidating. They can be cleaned professionally, of course, but it isn’t cheap.
When lenses are removed, the sensor area becomes exposed to ambient air and can collect dust. Moving parts inside the camera can create fine particles, too, and these can deposit on the sensor.
When dust particles fall on a sensor, they may appear as spots on images, particularly those taken with a small aperture.
Dust removal systems work by rapidly vibrating the sensor to remove the dust particles. In addition, many manufacturers now put anti-static coatings over their sensors. These systems work with relative efficiency though are not perfect.
All three DSLRs I’m considering have an effective integrated dust reduction system.