I find it hard to understand the purchase of military commissions in pre-modern Europe. The purchasers were usually the nobles, who were often unwilling to grant taxes to the Crown. Yet, when it came to military commissions, they were willing not only to serve (i.e. to shed their bloods), but also to pay for the privilege to serve? It sounds like a rotten deal.
They must have got something out of such purchases, in addition to the sense of knightly honors, right? What would that be?
On the other hand, from the Crown's perspective, how did the King make sure his military officers were competent, when such positions were actually bought?
The question is asked regularly. Here is a previous 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
A reply to /u/DavidPRNT
As /u/CChippy said, variations of this question has been asked before, and I think /u/CrossyNZ answers it most completely, but there are a few things I'd like to add.
In the early modern era, there was no fluid banking system. As the sizes of armies and the cost of maintaining them grew exponentially, there was pressure at the top level on how to fund military endeavors. The solution tended to be to roughly outsource the effort.
Among the middle and lower level officers, they had to take on the role of financiers at various levels. They were expected to arrange for equipment, wages, food, clothing, etc. to be reimbursed in the future.
But it was not all about them taking on risk that was pushed down from the top. There were very desirable privileges. The responsibilities of officers enabled activities that opened up the possibility for profiteering, by controlling the selection of vendors. They also profited from plunder. As such, up to the end of the Thirty Years' War, there was tendency to outsource military endeavors even starting from recruitment. And commission means that that officer has often-exclusive access to a particular region for recruitment. Higher level officers can expect rewards in terms of nobility, land grants, business grants.
So while in deployment, officers tended to have to carry some debt on their own. On one hand, this meant they were very invested in ensuring the unit under their command is kept in good condition, lest desertion and loss in war may result in severe devaluation of their investment or outright disbandment. What happens to the debts sustained when a particular officer is transferred, or fired, or retired, or died? The replacement officer has to take on most aspects of that debt, through a new commission. So, the commission enables financial continuity. Military commission literally means financial commission.
I wrote here on several examples in the early modern period. Two outstanding ones are Spinola and Wallenstein.
These debts at all levels cannot be under-estimated. The so-called "Swedish dilemma" at the end of the Thirty Years war refers to the desire of Swedish armies (and of course, their officers) to obtain compensation for their debts in exchange for peace in Germany.
Following the closing of the Thirty Years' War, and after the two Frondes in France, Louis XIV started a new model whereby the higher nobility (Nobles of the Sword) were successively weakened in privileges and in military ability. His state government took on more of the top-level military enterprise roles, while developing its own corp of middle and lower level officers. In this era, there were still the same responsibilities and privileges among these officers, although they were severely curbed and streamlined. These positions were still considered to be lucrative, as it opens up the door to social and professional advancement.