I love the sleep sack!! Some reasons I use them: they're safe, as I'm not comfortable with blankets for the littles, plus they stay on/with them whenever they move around. They act as a good sleep cue when only used for sleep times. They keep my baby from removing their pj's and/or diaper, and stop them from climbing out of the crib. Bonus: they grow with them, so you don't have to have many sizes!!
The list goes on :)
4 Season® Ultimate Baby Sleep Bag, Merino Wool, 2 Months - 2 Years, Earth