Leaving unmanaged areas in the garden gives pollinators great undisturbed spots to shelter, nest and hibernate in.
Many garden weeds are also important food plants for pollinators.
Many of our most colourful and well-known butterflies depend on stinging nettles for the growth of their larvae, including peacock, small comma, tortoiseshell, red admiral, and painted lady (see more on the Woodland Trust webpage). In total, there are about 40 insects relying on nettles in the UK and they are also good for a range of other wildlife.
Common ragwort is another weed with an undeservedly bad reputation. In the UK it is declared to be an injurious weed in agriculture as it can be toxic to horses and cattle when consumed in large quantities over a long time. However, in a garden situation it is perfectly benign and unlikely to be harmful to humans unless you are keen on eating up platefuls of this bitter-tasting plant. In fact, ragwort provides a veritable feast for over 100 UK mini-beasties, many of them pollinators, including several endangered species. One of the more recognisable is the striped cinnabar moth caterpillar which munches away at its leaves in July and August.
You can read more about the ragwort myths on Friends of the Earth website here.
Buglife provides a great summary of the range of mini-beasties which rely on ragwort here