The antigens carried by antigen presenting cells (dendritic etc) are drained through the lymph from the periphery into lymph notes, where various progenitors live. There they recognize antigens and proliferate. You don't want them to be in the periphery because they might miss antigens, and proliferating requires special structures, for example to avoid tissue damage.

So these lymph nodes are hubs where a huge diversity of progenitors is available to recognize any antigen, and which is able to see antigen coming from a significant area in the body. You can think of it as a scout discovering an enemy, bringing the news back to the local camp where all the officers can decide the best course of action.