Landing flap question
Enthusiast not a pilot so please excuse me if I'm asking something obvious. I would like to know why certain aircraft don't always use full flap on landing, I'm thinking in particular of the 737 which uses 30 deg of flap mostly? What are the advantages of this, and why wouldn't a pilot want to land as slowly as possible? Is flap 40 just used for particularly short runways, or in certain weather conditions?
Thanks in advance