They can improve security of the network. It also improves Management . Firewall systems are intended to be the single point through which all Internet traffic, both incoming and out coming is passed. Because the traffic passes through the firewall it can monitor information so that it has to meet its access control policy. A Firewall has basically two jobs, one to block information being set he other to allow information to be sent.
Internet access is provided by a router connected to the Internet.
The hub allows flexibility in a building by allowing PC's to be moved around and still can be connected to the network. A hub has ports. When a packet arrives at a port, it is copied to the other ports so that all the nodes on a network can see the data. This is common with email that get sent between different e-mail companies. A repeater is a device that is used to repeat a signal.