I am not very familiar with finance. Glazer is the owner of the club, so just like any business or publicly traded company, any money spent in the club either on infrastructure, player transfer or wages, is "his" money, right? How does it different if you borrow money from the bank to buy a franchise like a McDonald and use the revenue/profit to pay off the interest or debt?
The way you think it works is like how a "mom and pop" shop is being run - if it's a small scale family business, technically yes any money that comes into the till is "your" money. However, things become slightly complicated because say if some family member takes money from the till willy-nilly then it becomes difficult to reconcile actual business earnings if the money left in the till at the end of the day does not match up with sales. Also, as a properly run business you want to make sure that you make a profit - which means that you should make a profit (a.k.a profit margin) after factoring in cost of goods sold, rental cost, employee cost, ops cost etc. and it becomes absolutely crucial to make sure you know how much cost is going into the business v.s how much you are making after deducting cost.
Additionally, personal tax rate is different from business tax rates. Business tax rates are often lower (after taking into consideration the subsidies) so it becomes crucial to keep things separate between personal and business.
To ensure that things are being tracked properly, what happens then at any business that exercise proper accounting is to dis-associate the business from personal account. Whatever surplus a business has comes back into the business and may be used to either restock sold goods or potentially reinvest into the business.
Now, this is more crucial if the you have more than one business. You want to make absolutely sure that one business is not subsidising the other. This is not yet taking into account the complexity of each business having to pay their own employees and taxes.
Coming back to us - the way Glazers took a loan which is not based on his personal wealth and assets, but rather on the projected future earnings of Man Utd. If for whatever reasons the venture fails, Man Utd will be liable to return the loan, not Glazer. To highlight how ridiculous this is - imagine I wanted to buy Apple (the company). I took a loan from the bank based on how profitable Apple currently is and the project future earnings, and initiate an aggressive buy over of Apple using the loaned money. Any future earnings made by Apple is used to pay off the loans while I take a massive cut as the primary shareholder.
This shite is so ridiculous that EPL updated the rules around club ownership and buyouts to prevent future incidences.