If I sync with iCloud, it basically makes sure my device and the iCloud stay in sync - which means that if i delete a photo on my phone, it will delete it from the iCloud, and vice versa.
I want to free up space on my phone, and I don't really need the photos on my phone - I can just see them in iCloud.
If I turn sync off, I'm assuming I can delete the photos from my phone and they'll stay in the iCloud.
The question is, when I take more photos, can I easily see which ones haven't been backed up?