Why do we need antivirus software?

To protect your system against viruses 

It's a big business use the built in one window defender in windows 10

to make sure your pc doesnt get sick

Antivirus softwares are programs that help protect your computer against most viruses, worms, Trojan horses, and other unwanted invaders that can make your computer "sick." ... You must also ensure both the program and the virus signature files are up to date..

Antivirus software is an important tool to help prevent such attacks. Not every type of cyberattack can be prevented with antivirus software, but it can be a great asset when trying to prevent intrusion into a computer. For securing against the virus, worms, and trojan horse we use the antivirus software.

Unfortunately in this day and age, there are people who do not believe in leaving other peoples things alone. Therefore, they create programs, scripts, batch files and so on, that are designed to exploit weaknesses in the systems people use and to make money from what they can retrieve through their exploit. They also do it to be a nuisance and to basically give the middle finger to everyone.

to protect against unwanted intrusions of hackers in our system

To protect our device from virus, hackers and other stuffs

To protect our devices from malwares and virus and hackers

Because you don't have an Apple  device.