Creating connections is costly and it doesn't make sense to create a new connection for each transaction that might only take a few milliseconds. Managing database connections in a pool means that applications can perform database transactions in a way that avoid the connection creation time (The connections still need to be created, but they are all created at startup). The downside is that if all connections are in use, and another connection is required, the requesting thread will have to wait until a connection is returned to the pool.
