Seniority based systems are ‘I got mine, f u’ or ‘politics in action’ depending on how you look at it.

More senior employees have usually figured out how to get leverage on the employer over time.

Non-seniority are usually ‘cheapest is best’, or ‘do what I say, or else’.

Both have pros and cons for everyone involved. There is always some system though, even if it’s emergent.