Dreamcatchers- kind of a misnomer. They only catch nightmares. Good dreams slip through the gaps (they are sleek and svelte, if not particularly swift) to find their way into your sleeping head. Nightmares, big cumbersome brutes, tangle and thrash in the dreamcatcher, held fast through the night until the rising sun burns them to ash.
But that is the key. Only the sun can vanquish nightmares. If kept in the dark, they simply remain trapped in the web. Trapped creatures are desperate. Desperate creatures are willing to bargain. Just be very careful as you negotiate the terms of their release.