Copyright holders, along side YouTube (to use a well known example), have, within the previous couple of years, been cracking down on people using copyrighted music without permission. Tributes & covers of songs, of course, are somewhat "riding the fence" of legality, where the "μουσική χωρίς δικαιώματα" artists gives their due credit to the composer/lyricist in question and provides a disclaimer in their video description that says "this isn't my work, I don't own the rights to the present music," and that they generally get left alone, more or less.
Obviously, though, not all videos with music in them are covers or tributes. Other videos are sometimes sales pitches, or vlogs featuring meditation music. Whatever the reason for including music at all, it's something of an embarrassment for a video creator if they have to remove or replace the music in their video because the music was copyrighted, and the creator had their video monetized. And since not every content creator out there earns lots of money from their videos, they likely can't afford to pay the royalty fees required of them, so they're forced to remove the music.
Finding Free Music
But not everyone is a musician and/or lyricist with the ability to compose their own works. Nor does everyone have a musician pal at their disposal, whether that musician gets paid for ditties and jingles or not. So it's often up to the one creating the video to seek out either free music, or at the very least, royalty-free tracks. But where to find such music? And is the free music you'd find of high quality?
After all, it used to be that most stock music libraries didn't have music that was very good quality, and some music libraries are still like this if the people uploading tracks have used cheap equipment and synthesizers where the built-in digital sound banks aren't very accurately sampled & reproduced (this tends to happen with lower-end electronic keyboards).
However, because of the arrival of MP3s and more professional-grade composition & editing software, among other similar advances, most stock music libraries lately often have truly stunning tracks available in a wide variety of genres, so you're not stuck with just a few samples here and there of one style, and a lot of samples in another style, and nothing to speak of in any other genre.
What's the Difference?
Now, you might be wondering what the difference is between "royalty-free" and completely free, when it comes to stock music libraries.
"Royalty-free" means that you pay a one-time fee for a track and you don't have to continually pay royalties to the composer for the entire time you use their works, and there is usually some sort of usage license that detail how you can use the music once you pay the fee. Completely free music is simply that: It that has no upfront usage fee and is normally licensed under the "Creative Commons" license.
When you go looking for free music, you'll want to take a look at the types of music licenses available, as each licensing company has different stipulations as to how the music they need on offer are often used.
It's worth your time and energy to go looking for free music. It saves the effort and embarrassment of getting to get rid of an otherwise copyrighted track and replace it, plus you never know what amazing, original music are often found at reasonable prices.
rahul