Malware can indeed be a problem. To be quite frank, the majority of viruses don't even care to destroy your computer to actually do it any harm. The majority of the time it's something similar to what they call a network RAT where the primary goal of getting you to open a piece of software is simply so that they can use your network on something a lot of people call a botnet. Which is highly illegal but the majority of viruses are for that specific reason. Upon opening, it basically hi-jacks a portion of your broadband network and gets put onto programs which are not limited to IRC bots or other kinds of bots that they then use to attack people.
Long story short, they're basically using them to attack other people. That's the most common trojan. Others do actually steal your data, passwords, credit cards... etc. That's the second most common. I will say that one of the most uncommon are actually what they call worms that actually destroy your entire computer. Mainly because the hacker wants something out of it, and will rarely attack you for the sole purpose of harming your device.
Long story short, they're basically using them to attack other people. That's the most common trojan. Others do actually steal your data, passwords, credit cards... etc. That's the second most common. I will say that one of the most uncommon are actually what they call worms that actually destroy your entire computer. Mainly because the hacker wants something out of it, and will rarely attack you for the sole purpose of harming your device.