fladd wrote:What I still don't understand then: What is the benefit of having more analogue gain but also more digital headroom?
As Hugh says, the design choices can be more influenced by commercial than technical considerations - the end aim being to make something that can be sold at a profit and people will buy, rather than a technically perfect device. Cost engineering considerations will also come into play, which can include things as odd as 'we can get this part cheap because we buy lots of them for something else we make'.
On the wider subject of gain, in devices that take an analogue input and give a digital output, you are quite right to be a bit confused by the specifications (and Hugh is equally right to be annoyed by the the fact that half these specs are usually missing). At the end of the day, both signals represent the sound, but the magnitude of both is meaningless and only relevant to other parts of the same signal, apart from some conventions on signal levels that help us all join bits of kit together.
When the analogue signal is put into an ADC in the same device, the signal level (and hence the analogue gain required) at the conversion point is arbitrary and irrelevant to the outside world. It would be just as possible (but pushing what is sensible!) to have an ADC with a full scale input of 1mV as an ADC with a full scale input of 100V - the latter would obviously require huge amounts of gain in the analogue stage but you could/should end up with exactly the same numbers come out of the digital stage. The choices here a very much about what is easiest to do for the lowest noise.
As both you and Hugh have alluded to, the range of gain available is what gives you flexibility to cope with a range of source levels. But again, this is mostly a feature of normal designs. It is perfectly possible (again, not necessarily sensible or practical) to build an interface with zero analogue gain, but an adjustable input scale on the ADC - i.e. you could change the reference voltage for full scale from a few mV to a few V say. Give or take a few technical considerations, you'd end up with exactly the same data coming out the end!