Why is it important to study chromatin to understand cancer?
Most cancers involve, in addition to genetic changes, a whole suite of epigenetic changes that orchestrate changes in transcriptional profiles. Additionally, multiple epigenetic modifiers (Eg - EZH2) are associated with worse prognoses. 
There is also the fact that chromatin modification machinery is very highly mutated in cancers; examples include EP300 - a histone acetyltransferase, and members of the SWI/SNF nucleosome remodelling complex. The study of epigenetic factors needed for the continued transcription of oncogenes like Myc has also led to the formulation of potential therapeutic strategies (Check out some of the literature on inhibiting Brd4 to silence Myc in certain settings).