It just occured to me that I should explicate why I believe this:

Basically, it boils down to the Imperial government [at least IMTU] being a truly feudal entity. Each High Noble is assigned a specific region of the Imperium as a tax fief. Within each fief, each High Noble is responsible for collecting *and* spending all Imperial taxes and other revenues (including the Emperor's share in income from local Imperially-chartered entities). The basic distribution of this income (again IMTU) is based upon the medieval Terran division of military loot: the High Noble keeps 1/3, with the rest passing up to higher levels of the government. [NOTE: An important and often overlooked part of the post-Civil-War Arbellatra Reforms - again IMTU - was to remove local command of the Imperial Navy from the purvue of all High Nobles, with another reform being the assignment to these newly-independent IN commands one-half of the taxes which had heretofore been sent to higher government levels.]

Therefore Imperial High Nobles are not spending "someone else's money." The 1/3 of revenues which they get to keep is in most ways their own money, because one functional definition of feudalism is "government power as a private possession." Of course, the Emperor still legally owns all of the tax revenues and therefore expects that the noble will spend the share he gets to keep in such a fashion as to best benefit the realm. But the Emperor makes no attempt to decide in detail how this should be done, beyond requiring the maintenance of a list of certain fixed items of infrastructure (usually including a reserve naval fleet]. What percentage of the taxes each High Noble chooses to invest in improviing the area of the realm under his control versus on such things as iridium-plated bathroom fixtures and decadent balls is entirely up to him or her.

Of course, at any time the Emperor may call for an accounting of how "his" tax revenues have been spent: The Parable Of The Ten Talents: https://www.biblegateway.com/passage/?search=Matthew+25%3A14-30&version=ESV

High Nobles who have spend tax revenues wisely when called to account are rewarded [usually with larger and/or more lucrative tax fiefs]. Those who spend their taxes less wisely are passed over for promotion. And those who displayed extremely poor stewardship are "cast [sic] into the outer darkness. In that place there will be weeping and gnashing of teeth.’