Why do they often analyze lymph nodes for metastasizing cancer cells?

The lymphatic system is connected to the venous system. Hence, cancer cells can also travel to lymph nodes far from the primary tumor or to other organs or tissues in the body, where they collect to form a metastatic tumor.

There is a propensity for certain tumors to seed in particular organs. This was first discussed as the "seed and soil" theory by Stephen Paget over a century ago in 1889. The propensity for a metastatic cell to spread to a particular organ is termed 'organotropism'. For example, prostate cancer usually metastasizes to the bones. In a similar manner, colon cancer has a tendency to metastasize to the liver. Stomach cancer often metastasises to the ovary in women, then it is called a Krukenberg tumor.

According to the "seed and soil" theory, it is difficult for cancer cells to survive outside their region of origin, so in order to metastasize they must find a location with similar characteristics. For example, breast tumor cells, which gather calcium ions from breast milk, metastasize to bone tissue, where they can gather calcium ions from bone. Malignant melanoma spreads to the brain, presumably because neural tissue and melanocytes arise from the same cell line in the embryo.