Non-techie overview (I'm not by any means qualified to give any other kind) of Linux use. This is just the most common desktop use, though there are many server and other more specialist uses.

Your computer has an operating system, probably a version of Windows from Microsoft, less probably a version of OSX from Apple, less likely still one of the other systems, such as Linux. The first two are provided at a cost to you by their respective corporations. Linux systems are frequently available for amateur use at no cost beyond a bit of effort installing them, though paid-for support may sometimes be how a given distribution is funded. Installing is not usually that much effort, I have done it despite being a bone-idle non-techie.

Some Linux distributions (aka distro's) are developed by corporations, others by enthusiast groups, some even by individuals, each with their own agenda as to what their ideal system will do, what it will look like, and what resources it will need to do the job. One developer may, for instance, be fixated on keeping the most minimally-resourced machine functioning, whereas another wishes to have the most whizzy, eye-popping desktop, or perhaps the greatest possible selection of video editing software out of the box, and the greater the resources used the better. Enthusiasts tend to emphasize how inefficiently the big two systems are written, I don't know how true this is, but in both cases they seem to need ever greater resources while MS and Apple have good commercial reasons to want to keep you consuming, coincidence?

Choosing a distribution is a bit like choosing a vehicle - you read the specifications and reviews, test-drive a few, then go for the one you fancy. Some come with everything fitted, loaded and polished, others are more like a kit car. One good thing is that changing your mind usually costs nothing but time.

Keeping their system up to date is a job that some developers embrace with greater enthusiasm (and/or competence) than others.

So, reasons you might give Linux a go: any one or more of

You own an old computer you would like to keep going, but it hasn't the resources to run an up to date version of Windows. You feel like a bit of a challenge. You fancy building a computer but don't feel like giving more dosh to Microsoft or Apple. You want a machine that does just those jobs you have chosen for it. You want to learn some computing, you can build your very own version of Linux if you wish to, as some do, and both instructions and enthusiast forums are available.

I hope this helps a bit.