The team are quite happy to hear about these use-cases where you think the multi-column UI doesn't do as good a job compared to the grid, particularly if you can give some examples/screenshots, so that the UI can be improved. If you write in to customer support they'll collate the feedback and ensure it goes to the right people.
So, I went looking for an example of when the grid is useful, which is usually when the options include a mixture of off-peak returns and single-leg priced advances.
For this particular search, here is what each version offers me:

And I
was going to point out how the grid lets me look at that £85.40 super off-peak return that is my best bet if I want to travel out on the 0934, and see what return trains it will let me travel on - or how much extra it would cost to travel out on that one, and return on one that it wasn't valid on.
But I realised the newer system does deal with this (choosing an outward journey and seeing what it does to the return options) in an OK way, by showing additional costs in this manner:
What I think the grid lets me do, though, that the newer one doesn't, is look at a particular price point for a
return leg and then see what my relative choices for the outward leg are, pricewise.
That is useful when you are fairly fixed on the time you want to return, but flexible on the time of your outward journey.
As an additional observation ... I did notice that the grid system offers me that £79.20 lower fare if I return on the 1945-2324 option ...whereas the new system doesn't seem to offer me that.
This is all very complex though, and I'm guessing that the newer system decides that option is too slow to mention. I did have "include overtaken journeys" active though.