Do you mean when receiving the funds? That's for privacy reasons. See this[1] and if you referring to the funds sent to "another address" when you send some BTC, then those are called change addresses which you can also read more about here[2]
[1] https://support.ledger.com/hc/en-us/articles/360034336713-Receiving-address-changed
[2] https://support.ledger.com/hc/en-us/articles/360033802154-Change-addresses-for-UTXO-based-crypto-assets?support=true
That is exactly what I meant. Thank you for your good explanation and the further information. I will look into it more intensively.