Detection systems are what allow a ship to "see" what is surrounding it, without them a ship is almost completely useless.
Sensors are passive detection systems. They detect and alanyze all information that hits them, including light and radiation.
Scanners are active detection systems that send signals out to get more accurate readings and information. While they can gather more detailed information than the passive sensors, the signals they send out can give away a ships position to the passive sensors of another ship.
Sensors and scanners can be fooled or damaged or otherwise rendered ineffective. In these cases it becomes necessary to rely on the natural eye looking out a window to see what is outside. Windows on ships are incredibly thick to compensate for not having an armor coating.
The view screen takes all the data from the sensors and scanners and filters it into a natural light image. This allows it to act like a window with enhancements like a zoom, and the ability to show what would not normally be visable to the natural eye. But it can only show what the sensors and scanners detect, and they may miss things that are visable to the natural eye that the sensors are blind to, like stealth ships.