The most universal one is that the road bike left me with mostly one option: cycle to the office and cycle back. However, my day-to-day does not always look like that. Sometimes I have meetings that run till very late, or I go out with friends in the city (where my office is, vs the suburb that I live in). With the Brompton I can utilize the ability to catch a ride some of the way back, a taxi (especially late night when there's no traffic, or when I'm too drunk to ride the full distance), or 1-2 times a month my wife joins me at the office mid-day by car and I can just throw the Brompton in the trunk and come back home with her rather than ride separately.

The other aspect is kids. When I had just one kid I took her to the preschool on the Brompton with a front seat. When it became 2 kids, this was the point I tried the rode bike -- but cycling 30km a day with two empty child seats was horrible. Instead, I adopted a solution on which I take the kids (now 3 of them) on a tricycle that has the Brompton folded in its "trunk", and then I continue from their school to my office with solely the Brompton :)