It doesn't. Goutweed just uses up all the good in the garden, sucks nutrients out of the soil, doesn't produce a flower, needs tons of water and wilts easily in the sun. Basically a high-maintenance plant adding little to your garden.
Its also a greedy litle fucker that wraps its roots around any other plant slowly strangling them. This invader killed off an entire strawberry patch, all my Echinacea, bleeding hearts and Irises.
I've come to despise it and after digging more of it up consider it a perfect analogy for Wall Street; a living thing that it sucks up all the resources around it, doesn't produce anything of value and is impossible to get rid. Goutweed was brought to North America as an ornamental from Europe and Asia, probably for the gardens of Wall Street. Kill it when you see it.