The situation came down to ownership and control. When she signed with her first label as a teenager, she didn't own the master recordings of those early albums - the label did. That meant they could license her music to whoever, use it in commercials, alter it, whatever. She wanted her own work back, so re-recording became her strategy to own new versions and essentially make the old masters less valuable. It's actually a clever move if you own the rights to re-record.
The re-recorded versions sound almost identical to the originals at first listen, but there are subtle differences - slightly different vocal takes, maybe a bit of new production polish, and she's also added vault tracks (songs that didn't make the original albums). The big practical difference is that she now owns these new recordings outright, so she controls how they're used. Technically the old versions still exist and probably always will, but the idea is that her fanbase shifts to the new ones over time.
One thing people gloss over: this only worked because she had the leverage and catalog size to pull it off. Most artists can't just re-record their whole discography and expect it to matter commercially. Plus, the re-recordings won't hit exactly the same way as the originals did - there's something about a song's first version that fans connect with, even if the new one is technically "better." So while it solved her ownership problem, it's not like the original albums just vanished from people's playlists.