I spent most of my working life with same multinational.
At first we used a local travel agent in the town where our business was located, they were brilliant and always got us very good deals.We were their major customer and they realised that.
We then had to use the travel agent who had the contract to provide services to the company's European arm based in London, they were expensive but if we could show them a better deal they would match it because of a clause in the contract.After a year the deal was renegotiated and the clause was removed.Fortunately i was senior enough to be able to create a fuss about the extra expense on my budget and my VP took it up. At the next renegotiation the agency lost the contract and i was advised that(because of my vigilance!!) all air travel was being transferred to corporate travel at our headquarters in the USA.
Corporate travel then moved from expensive to very expensive and my travel budget almost doubled, i complained about it and was ignored. I then instructed all the people who reported to me to get a comparable deal from our local travel agent and if it was cheaper use it.The local deal was always much cheaper often 50% of corporate.After about 6 months i was hauled up to the London office and told that if i didn't conform to corporate travel policy i would be disciplined.My proof that my way was much cheaper was ignored and as i didn't understand the big picture i was wrong. I was told that the company received a commission from the agency used by corporate every six month's depending upon the amount spent. I then tried to get a credit to my budget for my share of the commission. I never did get it and i could never find out how much commission was paid although i was told,on the quiet, that it was nowhere near the amount i could save.
In short using corporate travel is expensive but they can be a powerful organisation in the company!