What is the significance of the last 5 digits of Base 58 WIF?
The last 4 bytes of the WIF format is a checksum. The details can be seen here: https://en.bitcoin.it/wiki/Wallet_import_format

That's approximately 5 characters of Base58 that are essentially redundant. This is so that errors in the WIF encoded private key can be detected easily. I believe some wallets will simply ignore the checksum if it's not correct.