When lignin is stored in the cell wall of plants, it is called "lignification". The lignin is stored between the glucose units of the cellulose, it comes to the formation of lignocellulose and thus to the formation of wood. Lignocellulose is very important for large trees, because without the supporting wood they would bend like a blade of grass. In addition, the tree would die of thirst, because the woody cells serve as a water pipe system that reaches up to the tops of the tree crown.
Explanation for children:
Many plants have lignin in addition to cellulose, which makes them look like a tree, because lignin forms the wood of a plant. Wood is very stable, which is why we make a lot of furniture out of it. One can imagine that wood is very difficult to break down. Only very few microorganisms can do this, e.g. some fungi.