In major cities here on the West Coast, particularly Seattle, Portland, San Francisco and Oakland (I'm not sure about Los Angeles, but I bet it's the case there too), a key threshold for showing a movement's power was whether you had enough people overall, and enough determined people, to block the interstate. Or, in the Bay Area, one of the major bridges.
Other times, knowing that large spontaneous, breakaway marches were likely to happen, smaller affinity groups had independent actions planned to coincide with them or make use of the cover provided by a giant crowd.
Again, apropos of nothing in particular.