In comparison to CpG islands, which are found in the regulatory regions of genes, and are prime importance in the transcription of the regulators of cell growth and death, repetitive sequences, also known as endoparasitic sequences, which are transposable elements, inserting themselves into any sequence they please throughout the genome, are thankfully in normal cells, highly methylated at CpG dinucleotides.
This, basically prevents them from propagating and disrupting the genome. The role of DNA methylation is thought to act as protector of genome fagility and activation of endoparasitic sequences (1, 2).
(1) Walsh, C.P. et al. 1998. Transcription of IAP endogenous retroviruses is constrained by cytosine methylation. Nature Genet. 20, 116-117
(2) Gaudet, F et al. 2003. Induction of tunors in mice by genomic hypomethylation. Science 300, 489-492