What could independent investigators learn from the first (known) copies of the virus from Wuhan, if they had (physical?) access to these samples?
I'll take a shot at this. It's my impression that most of this is political rather than scientifically grounded.  There are reasons to want physical samples of viruses (rather than just genomic sequences), e.g. if you want to do experimental (in vitro or in vivo) studies on a particular virus strain. (It might be theoretically possible to edit the genome or reconstruct the genome of an earlier strain, but it's much easier if you have the desired viral strain in hand.)
Based on this Financial Times article (which may or may not be accessible without a subscription/accepting cookies)

Mr Pompeo added the Chinese Communist party had still not shared virus samples with anyone outside China, impeding efforts to understand how it emerged.
A researcher at Wuhan Institute of Virology, who declined to be named, told the Financial Times earlier this month that China had not shared samples of the live virus with the WHO as the country “had already published plenty of information” such as the genetic code of coronavirus.

It does seem that almost all of the research on the origins of SARS-COV-2 is based on genome sequences rather than on phenotypic studies (i.e., in vitro and in vivo studies of actual viruses).
I don't think viral samples would actually be very useful (that's why I said at the beginning that I think most of this argument is political rather than scientific). In principle I guess you could use the viral samples from an early stage  do experimental infections in different animal species, but I don't know that any researchers actually want to do that, and I don't think there's been enough phenotypic evolution in the last few months that the early-epidemic viral samples would behave that differently from current samples ... it's also likely that researchers could study the effects of particular mutations (observed in genomic sequence) by modifying viral samples they had in hand.
To take an example from an earlier pandemic, researchers have done experimental studies in mice to show how particular mutations influenced the virulence of earlier flu strains. This is an example where it's useful to have an actual virus rather than just the genome - but in this case the virus was actually reconstructed from knowledge of the genome (&quot;two isogenic recombinant chimeric viruses were created with an influenza A/WSN/33 virus background containing the PB1 segment from the HK/156/97&quot;).