Simply put, there are constantly numerous processes running in the background on your computer that you probably aren't aware of. All of them constantly use your computer's memory to some extent. They read and write stuff in your computer.

Now, there are two types of memory on your computer, volatile, and non-volatile. Volatile memory gets deleted when you reboot your computer whereas non-volatile stays in place.

Generally speaking, your computer has two types of memory, ROM, which is your long term storage (though the term ROM, which is short for read only memory, is kind of inaccurate, as it implies that you can only read what is stored, but not write to storage, or in other words, you can access the file, but not modify it), and RAM, or random access memory, which is your working memory.

Now, ROM is always non-volatile, but RAM isn't. RAM is, however, faster than ROM, which is why both types are used. Sometimes, what you need is the ability to read and write quickly rather than the ability to store a file long term.

What's any of this have to do with these processes running in the background? Well, as they run, they may store data in your RAM that just doesn't get deleted. Afterall, even the most avid PC users rarely let their PC run continuously for more than maybe 12 hours, so leaving some files undeleted in a place that gets reset anyway after a few hours isn't a big deal.

However, in the odd situation where this does break something, you can just reboot your PC and that solves it. Same thing with many other software programs. They store various types of settings in RAM. Often times, these are settings are things you're not even aware exist, because they run in the background, without your computer ever letting you know they exist in the first place. If something goes awry, restarting the computer will often fix it for the same reason.