Let’s look at processed meat first. Processed meat is defined  as “meat preserved by smoking, curing, salting, or the addition of preservatives. This includes, for instance, ham, bacon, salami, and some sausages such as frankfurters.” Processed meat differs from fresh meat as it tends to be higher in salt and can contain preservatives added during the curing process which slow down the meat spoilage and reduce microbiological risks. You simply can’t make bacon and salami without them.
So what about nitrates and nitrites? 80% of dietary nitrates come from vegetable consumption and there is evidence that some sources of these in the diet support normal blood pressure . Nitrites and nitrates are used to preserve cured meats – you can’t preserve the meat without them. Salt has also been used to preserve meat for generations.
The prime reason to use such additives is the need to protect us against the microbe Clostridium botulinum. This bacterium produces a virulent toxin and the risk of death from botulinum is a very real public health issue. Recent research has also documented that nitrite used in processed meat products significantly reduces growth of Listeria monocytogenes.
There are theories, mainly based on animal studies, that overcooking meat, or changes in meat due to preservation, could explain links with bowel cancer but there is no consensus on this. In fact, an exhaustive review of the literature in 2010 by the government’s Scientific Advisory Committee on Nutrition (SACN)  concluded that eating up to 70g of red meat daily was consistent with normal health and adequate iron status. SACN noted difficulties in interpreting the inconsistent evidence base saying: “it is not possible to quantify the amount of red and processed meat that may be associated with increased colorectal cancer risk because of limitations and inconsistencies in the data.”
In fact, if you were going to prevent bowel cancer, the strongest evidence is for tackling smoking, alcohol, obesity and physical inactivity.
 http://cancer-code-europe.iarc.fr/index.php/en/ecac-12-ways/diet-recommandation/46-red-meat-processed-meat Hord NG et al. (2009) Food sources of nitrates and nitrites: the physiologic context for potential health benefits. Am J Clin Nutr 90: 1-10.
SACN (Scientific Advisory Committee on Nutrition) (2011). Iron and Health. http://www.sacn.gov.uk/pdfs/sacn_iron_and_health_report_web.pdf