A bulb has two terminals, one for sending the current to the filament and other is for receiving it back. Every electric component has two terminal one to send the current to device one to receive it back. It is not possible for any device to have only one terminal.

Also two terminal have important role in completion of the circuit. That is the most important condition for working of the circuit is that it must be complete it must not have any break point. Due to the concept of two terminal the circuit is always complete.

It is like a human chain as human have two hands one hand to connect with one person and other hand to connect other. At the end of the chain the first person and the last person join their hand. The person here are symbol of electric appliances, which have two hand as two terminal. If the electrical appliances are connected to each other like human chain than only circuit works.