Because trees take in carbon dioxide and release oxygen. Humans inhale oxygen and exhale carbon dioxide. It's a rather beautiful symbiosis, really. The trees enable us to breathe. And though trees don't breathe per se, our breath enables them to do the same.
That is such a beautiful symbiosis and so poetic too. It reminds me of a saying on one of my T shirts "though we exist as separate species, our fates are connected universally." It's so true.