Play mats stimulate babies to crawl and use legs, hands and arms, which helps build muscles. They also encourage laying on stomach which strengthens neck, back, and arms and also supports development of rolling, crawling, walking (collectively called of gross motor skills), hand skills, visual skills and even speech and feeding skills (stronger neck better supports the jaw essential for talking and eating).
Babies on playmats need to be monitored. While visual and cognitive development is better in an open space, monitoring baby constantly is a must.
Usually easy to clean and maintain.
Not an out-door item unlike most playpens.
Playpens allow you to leave your baby for short periods in relative safety. This is a huge plus for new moms and dads who are typically strapped for time.
Can double as a safe sleep spot (provided all basic safety rules are followed).
Many playpens come with built in mesh canopies that will protect baby from insects and sunlight.
Can be moved from one location to another without difficulty.
Many playpens come with in-built toys that help baby develop motor skills.