All bonsai pots have at least one drainage hole in the bottom to allow for water to escape from the pot and help prevent root rot. However leaving these holes uncovered will result in your bonsai tree soil escaping and creating a void under the tree. This needs to be prevented. This plastic drainage mesh is specifically for this purpose
This drainage mesh has a specific aperture (gap between the plastic strands) to allow for finer particulates and water to drain freely but retains the bonsai soil you require for healthy growth of your tree.
The aperture is also not so fine as to clog quickly, preventing drainage and very likely leading to the downfall of your tree.
Furthermore the mesh has a rigidity which maintains the shape, it does not sag like other mesh which is sometimes used.