M sand is produced by crushing a 10 mm aggregates to particle size finer than 4.75mm.
In direct production state, the crusher crushes the rock boulders, those boulders may consist of the soft rock which get crush to a very fine size and can be seen in a normal crush sand.
Producing a M sand (Manufactured sand) from a 10mm size already crushed aggregates gives assurance for almost very minimum amount of soft rock present in it.
So using the M sand for any kind of construction application is always better.
In manufacturing there is huge control over size of particles which can not be achieved in naturally occurring materials.