Fischer projections are best used to represent the straight-chain structures of monosaccharides.
A Fischer projection is a drawing of a 3D molecule as a flat structure. The chiral carbons are represented by crossed lines.
In addition to carbohydrates, it is useful for other molecules that have two or more chiral carbons.
Fischer projections are useful for differentiating between enantiomers, diastereomers, and meso compounds.
It is often easier to see an internal plane of symmetry in a Fischer projection.