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From Cities Skylines experience it’s usually to relieve traffic blocks by providing a direct path to areas/landmarks that have a higher than average traffic load. Not sure why they did it though.
That sounds like a reasonable explanation, but I’d have thought that Barcelona was laid out far before the advent of modern city planning.
This part of Barcelona pretty much was the advent of modern city planning.
https://en.m.wikipedia.org/wiki/Ildefons_Cerdà
Eixample was built in the mid-1800s iirc, and they did put some thought into its construction.