While it might be used to create the character for Star Wars: Episode IX, it might also be needed as part of filming additional scenes in Star Wars: Episode VIII that will explain why she doesn’t appear in the last part of the trilogy.
In addition, it’s highly likely that Star Wars will need to use a CGI Leia at some point in this process.
Carrie Fisher’s likeness was used in Rogue One: A Star Wars Story so it’s entirely possible that Disney is looking to secure her likeness for other projects entirely.
In fact, this may be the most likely scenario, as it would explain why negotiations are happening so quickly.