Question

why an electric bulb and an electric cell have two terminals

Answer 1

because one for positive terminal and one for negative terminal. But both the terminals are important

Answer 2

An electric bulb and a cell has got two terminals- a positive and a negative one . It has got two terminals as through one the current flows on and goes out through the other as it cannot pass through one side. In a closed circuit, the electron flows from the negative to the positive terminal.
So when it comes to the bulb the current flows in through the positive side, flows through the filament, lights the bulb and then flows out through the negative terminal.
I think the diagram below would help....