The only real reason for this merger is a corporate inversion[1] - a lot of companies are doing it. Corporate inversions allow you to move your HQ outside of the US and reap huge tax benefits.
Not true. It might be a reason but it's not the only reason, and certainly not the primary reason.
At the moment Burger King pays tax at a rate of 35% or thereabouts, on all of its income, no matter where it is generated. About half of its income is from the US, and half is overseas.
After the acquisition, it will still pay 35% tax on all of its income generated in the US, however it will only pay the local tax rate on its income generated outside the US. For example, this is about 15% in Canada, and 21% in the UK (there aren't many countries where corporation tax is above 35%... Japan comes to mind).
So the saving is the difference between the local corporation tax and the US corporation tax, on about half of BK's income - if the average worldwide corporation tax is 20%, so the difference is 15% and half that is 7.5%. So this is equivalent to reducing the rate of corporation tax in the US from 35% to 27.5%. While nice, it's not quite the "huge tax benefit" that you mention.
[1] https://en.wikipedia.org/wiki/Corporate_inversion