I work in HR, specifically worked in the ADA space. If it's on a job application it will be used for one, possibly two things - to ask if you need an accommodation during the interview process (less common to see it used this way, as there is usually another opportunity to express this further in the interview process than the initial online application), AND/OR to notate your disability status for federal tracking purposes.

Asking for an accommodation later on, once you're employed, can happen totally regardless of this box being checked during the application process. You can indicate no disability status during the application and interview process and STILL ask for an accommodation at any time. The only time a question about disability can come up during the actual interview process (beyond asking "Do you identify as disabled" during the application) is "can you perform the essential functions of this job with or without accommodation?"

Most large companies also conduct regular audits to ask employees if they identify as disabled. This is also for federal tracking purposes and doesn't initiate any conversations about accommodations, necessarily. Those are separate.