There are three main benefits.

- bigger seat, obviously. If you're over 6' it makes a big difference. I wouldn't do 13 hours in the CX economy seat.
- 2-4-2. If you're travelling with a partner you have a row to yourself. With 3-3-3 you always have a stranger by you.
- Smaller cabin and less chance of a screaming child in earshot, although there are often a lot of expat families on the LHR route in all cabins.