Victor Von Doom
Latverian Diplomat
Your question 'what is there to understand?' is the problem. There isn't a musical Rosetta Stone to allow you to 'get' certain songs. It's like saying 'what's so good about caviar? I don't get it- it doesn't taste that nice.' Then others are eating it, and loving the taste. They can't tell you how to like it. You might acquire the taste, or you might never like it.
The only difference with 'understanding' music is that there is measurable content that adheres to musical rules about complexity. You don't have to like it because you understand it, but there is something to understand.
I think the main problem here, though, is terminology.
People aren't clear whether they mean musical understanding, or an actual, tangible message.
There is much more going on in a Tool song than a Backstreet Boys song- musically. It's not a special hidden knowledge, like some kind of treatise; but, it does require a more complex understanding of music itself. This understanding can be implicit, and can be felt on a purely abstract level- in fact it generally is. The most obvious example of this is when someone says 'that song is just noise' when it's clearly not- it has distinct melodies, harmonies, structure, motifs, and so on. That's a lack of attention.
You can listen to Tool 50 times, and still get that 'oh yeah!' moment. The Backstreet Boys have the amazing quality of being understood even before the song starts.
Think of different style of comedy.
The Backstreet Boys:
YouTube video
Tool:
YouTube video