[Hayseed (OP)]

An electric field, a magnetic field and EM emission has but one source.  Charge.  An electric field, magnetic field and an EM emission can only effect one entity.  Charge.

An electric and magnetic field have density.  And a charge field moves, it rotates, so now the density has momentum.

Field density momentum...........is mass.  That's what mass is.   Mass is field density.  We refer to the mass as the density, because the amount of electric field, e, and the tangential velocity of the charge is constant.  The density is what changes, and this is done thru contraction and expansion of the charge rotation.  That changes the mass.

One can also scale "mass" to the RPM or frequency of rotation of the charge.  So the "mass" of an object is not only the number of particles, it's how fast they are spinning.

Particles can only spin at certain RPMs.  This is the mysterious "quantum effect".

There are two physical reasons for the set of spins.  The first one is geometrical.  This will allow the spin to change only at certain structural steps.  If this were the only condition, the levels could be changed in discreet equal amounts.  Still discreet, but in the same amount of step.

The other condition....is the balance in field density. This gives the odd steps.  Sorry to get into the weeds, but you seem to question the purpose of emission.  And to understand why we have emission, you need to understand that a charge will only absorb energy in certain amounts.   AND......if you put more into that charge, than it can except.......it will excrete, or cast off, that extra field momentum.  EM emission is an energy excrement.  A super quick excrement. So emission is the result of the quantum condition.

Physically, EM is the field trimmings of a stable charge.  There is another method of emission.  And that is the physical dislocation of the charge alignment.  And that is another weed story.

All emission is a loss of energy and angular momentum.  EM is dissolving energy into space.  An EM emission has field density.......so a EM emission has mass, temporarily.  It dissolves.

When EM is adsorbed by a charge, it empresses a torque on the charge.  That torque is not dependent on the intensity of the EM.  It's only the rate of torque(frequency) of energy to a single charge.  The intensity sets how many charges get torqued.

Therefore, the mass of a wave, is different than a mass of a photon.  An EM wave is the largest structure in the universe, a photon is the small part of it, that hits a charge.

There has been a constant mass and energy loss since the beginning...slow and cold now, but in the past it was a raging furnace.