It wasn't until after the Crimean war that the practice of purchasing officer commissions in the British Army ceased, and they moved towards a meritocracy for advancement in the military. What was the justification for the practice of purchasing/selling commissions? Was it considered a means of fundraising? Or simply a means of ensuring that only the nobility would control the armed forces? Was their any response when Revolutionary France started to shift to a meritocracy in the military?
While previous questions don't address the precise points you are asking here is a previous archived question with answers including a link to a detailed study of the benefits both to the individual Officer and to the Crown of the British system
https://www.reddit.com/r/AskHistorians/comments/1hnrbx/what_were_the_effects_of_the_purchasable/
and another, similar question, here