It might seem like a Buckle Me Baby Coat works the same as a regular coat but they are really very different.
When you unzip a traditional coat the puffiness of the coat has to go somewhere - usually behind the chest and shoulders. The real test shows at home when you strap your little one into the car seat and adjust the harness with no coat on then take them out without loosening the straps and try to put them in with a regular jacket - it will be way too tight and impossible to get them in without loosening the straps.