The value of things found in a swamp worth fighting for depends on:
1- the size of the swamp.
2- the location of the swamp.
3- the information/technology level of the people in the world.
These are the things which are worth fighting for, for a modern day swamp and modern day technology level. Edit them according to the technology level of the people in your world.
1- Rare/Valuable Fossils
La Brea Tar Pits are one of the richest source of highly valuable fossils in the modern world. Thousands of fossils have been discovered and excavated from these tar pits and thousands more are in the process of excavation. Notice that these tar pits are solid ground now-a-days, but in your world, you could have the fossils buried in the depths of a (large) swamp.
2- Route To Ore/Mine
There is a goldmine in an area and long ago some medieval ruler started to dig it up for gold and later had to stop work because of a war or geographical changes. The mine is still completely intact underground and there are large quantities of gold still present there. On the entrance of the mine, now there is a large swamp! If two or more tribes/groups/companies know about this, they will definitely like to invest their resources for claiming the ownership of the mine.
3- Home To Rare Migrant Wildlife Species
There is a species of very large and beautiful bird which is very rare. These are migratory birds and migrate between two continents. In this continent, these birds only nest around a specific area for laying eggs and settling for a few months. This area is in the form of a swamp. The feathers of these birds are highly prized as ornamental objects and nobles are willing to pay high prices for buying one of those birds as a pet. Once again, different tribes/companies would have their eyes on the ownership of that swamp and would not hesitate fighting for it.
4- Water Properties
Let us say there is an ore of some rare mineral under the swamp and the water of the swamp has immense medicinal properties (cure of some otherwise incurable disease). Even with the technology level of today, you cannot produce that mineral compound easily in the lab and the swamp has large amounts of that mineral dissolved in the water. I'm sure pharmaceutical companies would not mind waging proxy wars in the area for claiming ownership of that swamp and its healing waters.
5- Historical/Religious Importance
This is self explanatory. The swamp might have been home to some saint or some mythical creature in the folklore and having ownership of it is believed to bring goodluck and prosperity.
6- Important Trade Route
Let us say that the area around the swamp is an important trade route and any village/tribe who has control over the swamp can put their war boats/canoes there and tax all caravans. Every tribe/city around that swamp would want to have their control on it and would not mind going to a war for it.