Epigenetics are the "markers" that determine which portion of our DNA is meant to be used at a given time, by a given cell. Since each cell contains a complete copy of the entire organism's DNA, not all of that DNA can be used simultaneously, particularly if a cell is attempting to be specialized in a particular role in a multicellular eukaryote, such as a skin or bone cell.
The only DNA that cell should be using is the DNA relevant to its specialization; the skin cell would "deactivate" the bone genes, eye genes, and so forth. Since actually cutting out this DNA and discarding it is too risky, the genes are instead physically blocked, or made easier to access, if they contain relevant information, so that the cellular machinery involved in DNA transcription is unable to access it.
Technically, epigenetic markers should not accumulate over time; they should be laid down early in the development of the organism and then, for the most part, preserved as the organism ages. When that skin cell undergoes mitosis, the two subsequent cells should also be skin cells, with identical epigenetic markers identifying them as such. If epigenetic changes actually accumulate, this means that more DNA is being manipulated over the lifetime of the organism, and that cells should become increasingly specialized, which we haven't observed.
It's more likely that epigenetic changes accumulate by chance, in the same way that cancer is increasingly likely over a longer lifespan due to the increased number of possible causal events. Epigenetic changes may accumulate due simply to mistakes in the transcription process, or a variety of other errors that negatively impact the fidelity of cell replication.