It's like why reinforced concrete is stronger than bricks, because reinforced concrete has steel bars in it. In layman's terms, when the mortar is to be cracked to release the stress during the drying process, the wire mesh has been connected and fixed with the mortar, limiting the stress release of the mortar, so that it is completely solidified, and the solidified mortar is self-contained.

The wire mesh is like the bones in the mortar, connecting the loose mortar into a whole and improving the tensile strength of the mortar.
Also, consider the connection interface factor. The expansion coefficients of concrete and blocks are different, and temperature cracks often appear at the junction of the two materials. The wire mesh is used to limit the development of cracks.

The picture below is in the floor heating project, ready to lay barbed wire to fix the floor heating, accelerate heat transfer and prevent concrete from being heated to cause cracks.