Why is “baffle” the standardised measurement setup for speakers?
The baffle measurement is standardised in the IEC 60268-5 , even though in everyday applications, no baffle is present. So why does the industry rely on this particular setup?
There is one main reason: it is universal. The baffle is basically just a massive wall which separates back and front side of the speaker. The IEC-baffle is defined in a size that allows thinking of it as being infinitely large. This assumption allows viewing the two sides of the membrane (front and back) as acoustically completely separated.
Other alternative setups are: measuring a speaker with no housing (purely the speaker) or measuring the speaker with a speaker-housing (aka, box). The first will produce a very strong acoustic short circuit which is dependent on the speaker size, making significant measurements very hard. The second option will introduce two additional influences which will directly show in the measurement results: The geometry of the housing (radiation influence, edge reflections, standing waves etc.), and the simple size of the back volume. The latter introduces an acoustic spring to the speaker which reduces the compliance of the moving system and therefore reduces the SPL.
All in all, it is fair to say that for generically valid specifications of a speaker, it must be done in a baffle with no added back volume. This will produce the most reliable and versatile data, which an acoustic engineer can then extrapolate to any application. It is the setup, where all additional acoustic influences are completely avoided or at least best suppressed.
It is also possible to add a back volume onto the speaker backside during a baffle measurement. This makes sense if the speaker is specified to be always used with this particular back volume. Still, this baffle setup is better than having a speaker in a box in open space, due to the additional influences like mentioned above (influence in radiation, edge reflections, etc.).
 IEC 60268-5: Sound system equipment – Part 5: Loudspeakers, 2003