We need a virus detection system because virus is a thing which can spoil our whole computer or mobile phone. it can slow speer of our phone or computer. it can damage them. it can even corrupt the important data in our device. it can crash our apps or important applications etc. the drawbacks are that mostly they are not much affective and didn't properly clear virus from our devices. as I said first-
..your device will become slow...
the applications you have opened may stop even when you are running them.
so these are the things with which you can identify that you are having virus in your devices.
there is importance of backup for a computer system because by doing this you can save your important details or data if they get corrupted or deleted due to virus. mainly virus gets into your device through internet. like when we open the links send by company in our message boxes. that's why if you want to keep your device safe don't directly open links that comes into your inbox.