The first and most obvious answer to this is relatively simple. What is the purpose of the lymph system and the nodes that it contains?
Humans have approximately 500-600 lymph nodes distributed throughout the body, with clusters found in the underarms, groin, neck, chest, and abdomen. They act as filters or traps for foreign particles and contain white blood cells. Thus they are important component in the proper functioning of the overall immune system, and themselves part of that system.
When a lymphocyte recognizes an antigen which has responded to an immune system threat, B cells become activated and migrate to germinal centers. When antibody-producing plasma cells are formed, they migrate to the medullary cords. Stimulation of the lymphocytes by antigens can accelerate the migration process to about 10 times normal, and in that migration to the collection points of the nodes, result in characteristic swelling of the lymph nodes.
They are in the simples terms, part of a group of mechanisms that draw pathologies, and the detritus of our immune system battles, out of our bodies. Most do not realize this, but this entire lymphatic system is constantly filtering and removing the negative from us. Even the tonsils, now so often a place of primary/original location for SCC of the posterior mouth, are themselves part of this lymph system (this includes lingual tonsils and tonsilar tissue on the base of the tongue). It is logical for the detritus of the battle between our immune systems and pathogens which it protects us from, even common bacterial infections etc., to drain into the lymph system for further neutralization and removal from our bodies, enlarging the nodes in the process. The cervical nodes of the neck are the closest location for oral cancer to metastasize to because it is their job to be collecting this cast off material in the first place, as the immune system response in our mouths produces by-products that need to be eliminated.
In another post, some asked what might a primarily malignancy do besides this, and the answers are primarily two. One is that it might stay localized and invade more deeply into the nearby anatomical structures, into muscle or osseous tissues. Though in the long run detritus from this process will likely still end up in the cervical nodes and in extreme cases drain from the body through open fistulas. If the primary disease is near major circulatory system components, it may become removed from the loco-regional area via the blood, transported to other vital organs, which is of course how oral cancer kills us, usually not in our mouths. Primary means of metastasis of the disease from our oral environment is either the lymphatic system or the circulatory system which are of course each connected to the rest of our bodies. Single or multiple major vital organ malignant involvement is end stage oral cancer development.
All this of course an over simplification of what is a very sophisticated system, and entire books are written on how all this operates.
