The reason someone would want to buy old Google voice accounts is obvious: resell them. And potentially make more money than initially paid. Users can purchase an account with a working phone number and then use it as a “new” main number. Simply forward any incoming calls to the now-secondary account. Which then dumps all messages into an email that can be monitored. Users would not be able to download them directly from Google. So be sure to check the voicemail every now and then and download any new ones if needed (save as MP3 files).
The other way is to buy old Google voice accounts with fake names attached. Which tend to fetch a higher price than real numbers, because it is much harder to use one of these accounts themselves. And sell it on later, especially without giving away personal information in the process. Such an account can be attained easily.