A Pack N Play playpen can be used as a crib for the baby to sleep in especially when you're traveling.
I've used a Pack N Play as a crib when I was traveling with my babies and they worked great.
However for long term sleep use I would recommend using a regular crib that has a more comfortable sleeping mattress.
The pad in the pack n play playpen is hard and not that comfortable.
So your baby will sleep much better in a crib with a regular baby bed or crib mattress that is soft enough but also firm enough.
Some Pack N Play playpens also come with a bassinet attachment and some have changing tables attached as well.