Nothing against well written and beautifully delivered lyrics, but most songs are about a cheating BF/GF or a hard to seduce love interest. When you are listening to a lyrical song, your brain has two options. Either it treats the song as a background noise, or it follows the lyrics and your mind drifts in that direction. But do you really need cues from a song on; in which direction you want your thoughts to flow?
Instrumental music is a completely different beast. Instead of listening to complaints about an abusive BF/GF, or a hot love interest making the singer turned on, you enjoy the pure beauty of sound. The tune becomes a landscape on which your thoughts can dance and flow like a river.
I call it mindful listening. Put headphone on, close your eyes and experience pure music, just the beauty of sound. If you are amongst the 5% who are lucky eough to be able to experience synaesthesia (visualise music), you discover a wonderful new dimension of beauty. But even if you are not, there is nothing more relaxing than a blissful instrumental tune.