The sleep sack is a blanket that your baby can wear. Like a Snuggie but for babies. If you don't have the skills to pull your blanket up when you move in your sleep, then the sleep sack is a blanket that stays put.
Moving and restlessness and leg slamming and rolling are all part of the settling and self soothing process for babies. When I'm falling asleep I toss at least 3 times, scratch my nose at least twice (my hair tickles my face), I adjust and re-adjust my hair to get it out of my face and off my neck and wiggle around a lot. Self soothing is an active process and when you are done, you want your blanket in place.
"But what if it's too hot for a blanket. Can't you just crank up the temperature so you don't need one?"
The body temp naturally drops when you fall asleep so if you don't have a covering, when you wake between sleep cycles, you will be more aware and it's harder to get back to sleep. It's easier to sleep in a cool room with a warm blanket/sleep sack then it is to sleep in a warm/hot room with a light blanket. The sleep sack is better than a blanket because you can't kick it off. Have you ever fallen asleep on top of your covers in the middle of the afternoon, fully clothed, totally comfortable and as you are drifting off to sleep, grab the comforter from the other side and fling it over yourself. (Climbing in would be too much effort in your near sleep state) Your body temperature has dropped you got cold. In the heat of an Arizona summer, don't you wish it was cooler and you could climb under a blanket instead of sweating on top of one? A cool room with a blanket is just more comfortable.
When your child is able to pull up and adjust his blanket on his own, that's a good time to switch to a blanket. 2-3 years old is typical. If you have transitions to a toddler bed and your child still doesn't have that skill, a toddler sleeping bag is a good idea. Not a nylon camping kind but one made of quilting fabric that stays in place when you roll over. If you are crafty, you could just fold over a heavy, washable, quilt and put in a zipper.
To help your child learn the skill of pulling up his blanket, I suggest you read my blog about that. http://getquietnights.com/2014/04/blanket-races-help-children-sleep-through-the-night/
There are different types of sleep sacks. Swaddle, covering hands, hands out, feet out etc. You have to find one that works for you. I have heard great things about the Zipadeezip and the Pekemoe and the Halo and my personal favorite is the Moon Cocoon.
We just want our children to be comfortable and sleep well.
A sleep sack may be just to tool to help improve your baby's sleep. If you think you need more than that, Call a gentle sleep coach to give you gentle sleep guidance and solutions to keep your baby's infant mental health the priority.
Tracy Spackman is a Certified Gentle Sleep Coach helping families all over the world. Join her gentle sleep support Facebook group called Sleep Sisters - Get Quiet Nights.