If your battery is in good shape and just run down from leaving something on (and I'm still amazed that some modern cars from brands that supposedly paid attention to even the smallest details don't have any kind of anti rundown protection) then one of those jump start kits will work, but if you have a battery that is having issues or very cold weather then they are seldom powerful enough. Jumper cables can get you started in those situations.
I've seen jumper cables used to work around a failing started solenoid as well.