Sadly the answer to this one is usually that they are not good for a whole lot. Many antivirus companies have been caught time and time again, brokering backroom deals with malicious players, to let through their information stealing software, for a fee. This doesn’t mean that all antivirus software is inherently bad though, it may just mean that the paid versions are no better than the free/pre-installed ones.
One of the big issues with antivirus software is that tests are run by many magazines every year to compare the effectiveness of different antiviruses, when in all reality they are comparing apples to oranges. These tests are conducted by infecting a computer with several hundrede viruses and then running the antivirus software to see how many it can remove. Two problems arise from this, you get a count of how many viruses it can remove not which ones and these are only out of known viruses.
The reason why these two things are a problem is because you do not get to know which area each antivirus is strongest in or which type of viruses it is most effective at removing and you still only get a statistic on current, known issues. This does not mean you shouldn’t have any antivirus at all though, just like vaccinations, if we all have it we are less likely to get infected in the first place. It however means that as long as an antivirus doesn’t fail these tests miserably, any of them will do, which also by extension means that you do not get anything extra out of a paid version or a paid antivirus.
So what would my recommendation be for using as an antivirus, just so we can make choosing a bit easier for you? Due to the things I have mentioned, my personal opinion is mainly based on which antivirus gets least in the way of daily activities. For most devices Avira actually makes a very good free product and it can be installed on everything from Mac and Windows to iPhone and Android. A little side jump is that Windows actually comes with a very good product built in that is called Windows Defender.
For those of you out there that are a bit more adventurous, ClamAV is also an excellent product and it is truly free (it is opensource), but it may take a bit of effort to install on a Mac and you have to manually run it.