You have given the Job Description (JD) for the position of secretary and your question is how anyone can work for 24 hours? Your another question is that if the labour laws permit 8-hour work per day, then why this company is asking for 24 hours?
Technically you are correct. Nobody can be employed for 24 hours. Nevertheless, the company is ready to provide food and accommodation. It appears that the various tasks related to the job crop up at odd timings also hence this position requires somebody who can be available "on-call".
The position could be similar to the rector or warden of a hostel. For the safety and security of the hostelers, wardens make round to the hostel at odd timing. It could be late night or even early morning. Of course this could involve some strain but then nature of job is such that the person who takes up this job should be in proximity.
Hope my justification is acceptable to you. Nevertheless, other senior persons may also give their opinions.
14th June 2018 From India, Bangalore