The big advantage of multiaxials (=several layers of UD stitched together at different angels) is that it reduces fiber crimp (=in lament terms cloth stretch). All load carrying fibers are flat alongside each other.
When a load is applied they take the load almost fully.
Fibers in a woven cloth are woven over and under in a wave like pattern. When the load gets applied they tend to give a little before bearing the load.
This reduces also their bond within the resin matrix to some extent.
The other advantage of multiaxial fiber complexes (and woven roving) over chop strand mat is that they can be wetted out with catalysed resin before laying them down.
In the case of mat that is only possible to some extend, because the resin breaks down the fibers within the mat when applied.
Chop strand mat (=CSM) is good for low loaded parts
with complex shapes. When the binder is broken down the fibers can easily align themselves with the surface underneath.
CSM should only be used with Polyester or Vinylester resin as it contains the solvent to break down the binder.
does not have that solvent.
Woven roving and uni or multiaxials are best used with Epoxy
or Vinylester to get most out of their superior mechanical properties.