If the company is large enough the HR chain, the hiring manager, and the people involved in the interview will be completely different.
Even if they are both being advertised at the same time, they could be on completely different schedules for filling their position. One could fill it in a week, the other could take 3 months.
The different departments could have slightly different work locations, which could mean that the pool of candidates for what seem to be identical jobs could have few overlaps.
If one department makes you an offer, they will ask that you remove yourself from consideration for other job in the company. They don't want to be bidding against themselves.
For many companies it is far easier to apply for the 2nd, 3rd or 4th opening because you have already completed the hard parts of uploading the information from your resume into their overly complex application webpage. This ease of being able to apply to multiple positions encourages you to do so.