This is an excerpt from the sheet music for a song by Eminem. What do the cross notes mean, and what is their purpose? Why not use normal notes? (I see this frequently in hip-hop sheet music.)
They refer to non-tonal/non-harmonic sounds; be it drum sounds (these sounds don't follow a harmonic structure), or dead notes on a string instrument, or, as is the case in this example, rap (the rapper speaks the words without tuning them to a specific pitch).
This is useful for notating rhythm parts that don't really have a pitch. It's used instead of "normal notes" because it would be misleading to notate the example as, say, a B if the performer isn't supposed to sing a specific pitch.
They are actually called "Ghost Notes". In Wikipedia you can see a few examples of parts notated with these notes.