Quick answer
Reverb, along with other marketplaces like eBay and Amazon, need all items listed in “Brand New” condition to have valid UPC data in order to keep ourselves in line with Google Shopping’s requirements.
Ultimately, we expect the change to have a positive impact on your sales since it’s going to allow us to continue pushing your listings to Google Shopping as well as make your products more searchable within Reverb. Additionally, we're hoping to tie in UPCs as means to aggregate and surface your listings faster in the future.
Further Reading: