It is an accounting gimmick that makes your bottom line look good dynamically. They try to get other people to pay them quickly while waiting as long as possible to pay their own bills.
As a contractor working for a large company, I have to deal with that constantly. I have to threaten to leave and get them to treat me as a special case where I get paid on time. Every time they have a changeup in their accounting department, the new schmuck desides to sit on my invoices to make himself look good and the cycle starts over again.
It basically boils down to them not paying you until they have no other option. It is immoral and unethical, but it makes the accountants look good.