Calligraphic styling.

It's an extreme form of inktrapping.
It could improve colour of text.
It's a chirographic vestige.

Have a look at what Lirico does. The gaps fill in at text sizes.

It can be really pretty (reason enough, IMO).

Also, it could deviate forms from
each other (for improved readability).

hhp

The “a” in Eras and Friz Quadrata relates to an archaïc letter form.
Uncial perhaps?

@hrant: do you refer to Scala or do you speak generally? Because I have doubts about that 6/9 pair, since it's basically the same rotated form.

No, not Scala - just a general possibility.

hhp

In the case of Scala, opening up the counters definitely adds to its intended use. Martin Majoor conceived the typeface at the end of the 1980s with low resolution output devices in mind (such as the 300 ppi desktop laser printer). Hence also the slab serifs. So, a kind of ink trapping, indeed — or rather “pixel trapping”.

But the open counter of P has a much older, palaeographic origin. (Deviate the “Roman” (= Old Italic / Western Greek) Π (Pi) from the Greek Ρ (Rho).) Cfr the Trajan capitals, most Garamont interpretations, Palatino, &c.

That doesn't work because -like any other small feature-
the gaps will be rendered differently depending on size,
closing entirely below a certain threshold*, completely
changing the character of the design.

* And this threshold will furthermore
vary depending on the character since
the size of the gap varies.

hhp

Exactly. But with lighter/less pixels.