Protection against malware
The first and foremost task of an antivirus is to protect you from malware like viruses, trojans, ransomware, and more. It will make sure that no malware will ever get on your PC and warn you, if it finds something suspicious.
Most antivirus tools to that well enough, but some are just plainly better than others. Take a look at the latest comparison tables to decide for yourself.
Removing Malware is the second task any antivirus worth its salt should be able to handle. If you were careless enough to not protect your PC and somehow got infected, an antivirus is there to save the day. It will scan your system, internal drives, and any attached drives to make sure the malware is found and destroyed.
Once it’s finished with its work make sure to leave the antivirus on your PC: It can protect you from future malware infections.
While a antivirus can never take over the role of a good VPN most of them have mechanisms that will help you to protect yourself against being spied at. Be it cookies, spyware, or webpages that want more than their fair share of information: a good antivirus will have your back.
Time- and cost saver
Without an antivirus it is easy to fall prey to ransomware or similar malware. While installing an antivirus after an infection can help with a lot of evil software, ransomware will lock your PC down. That means that if you don’t have a backup you’ll either need to pay up (and not get your data back) or reinstall your system.
In both cases you’ll end up spending lots of time and sometimes even money to get things back on track. This can easily be avoided by using an antivirus.
Peace of mind
Do you belong to the people that claim they don’t need an antivirus because they are very careful when surfing online and opening mails / files / etc.?
That’s all perfectly fine, congrats to you. Everyone should be as conscientious when it comes to their online behavior. Nonetheless: No human is infallible. Without an antivirus danger lurks behind every link and download and it’s highly likely that you’ll get infected at one point. Why play Russian malware roulette if you just install a free antivirus tool? It costs you nothing and will give you a fallback if your common sense should fail you.