Basic magical theory is as follows:
Magic is the use of mana or other energies (as appropriate) to achieve lasting effects. This can be as simple as transforming mana, a highly malleable energy, into another kind of energy (e.g. Kinetic, Heat, Electrical) or as complex as creating portals between realms.
The inherent need to justify use of magic exists because "I say so because magic" doesn't mesh well with the inherent logic required of combat - strict adherence to time, to detail, and battle strategy are all essential for a good combatant, and each of these things requires use of logic in one form or another.
The need to justify is not, however, as deep as advanced physics. Even a rudimentary explanation to justify the mechanics of the spell on a very simple level are fine - and certain types of magic (e.g. Elemental magic) have inbuilt rules that allow them to be used without such justification (as such justifications are considered a priori).
If you use elemental magic on a highly complex level, you would be expected to justify the complexity as necessary, however.
Now, the justification becomes more important when you are using magic that is more complex than using the basic rules. As long as you have a mundane rationalisation for how the spell works, it should be fine. Certain actions are taken to be inherently known (e.g. Creating a portal), but will again require additional rationalisation based upon the complexity of the magic being used.
Provided that you think of magic as an advanced and highly malleable energy, this system is very easy to follow and makes a lot of sense. Justifying things as "because Magic" does not make a lot of sense and is inherently unsatisfying to deal with.
