Reindex? Do you mean rescan blockchain since the index for UTXO should be exist?
If the downloaded files are already pruned, then it wouldn't be possible for the user's wallet to be imported and be able to see all the transactions without the client reindexing.
You're right, i forgot Bitcoin Core need to obtain transaction history, not only list of UTXO. But i think "redownload whole blockchain" is more accurate than "reindex"
Why should one rescan the entire blockchain?
Usually you do it when open existing wallet.dat or import private key/address to obtain all transaction history. On Bitcoin Core, you use command
rescanblockchain ( start_height stop_height )
And what does reindex do?
Reindex means re-creating index of UTXO from existing blockchain files. Usually happen if files inside
chainstate folder is corrupted/removed.