One of the reasons that I recommend a helper program to whatever antivirus program you have is because of malware and spyware that gets missed. If you use something such as SpyHunter 5, it adds to your protection. People like different brands of antivirus programs. There is a wide variability of their effectiveness because of several factors. One is the virus definitions they have. The program needs to know what to look for. A big company will be up on what is coming down the Pike as far as viruses are concerned. They then do updates to have your software look for it. Depending on the operating budget of the company, your updates may come in slow or fast for the latest threats.
Another issue is the actual engine that runs the antivirus program. Not all of them are the same. Not all of them look for the same things the exact same way with the same level of effectiveness. The last things is how lenient or strict the antivirus program can be. Some are so strict you cannot do stuff that is perfectly normal and safe to do. It confuses regular computer users so they tone down the protection to not get the warnings. Then that is when stuff slips through.
Another bit of software that is running in the background and constantly on the lookout for stuff antivirus programs typically miss gives you that extra bit of protection you need. Even if you do not do any banking or anything serious with your computer, I would reason that you do not want to lose pictures and other things due to a virus or spyware destroying functionality. If you do banking and other things where sensitive information could be compromised, you certainly want extra protection against spyware on your computer.