It is not mandatory but it is recommended, reasons been that you don’t know whether the seed phrase is already compromised and using it on another device or wallet doesn’t save it from that danger.
Secondly it is highly recommended for closed source wallet because you don’t know what the wallet might have done with the seed phrase since it’s source code is close, creating a new seed phrase on electrum is better since it is open source wallet.


Yes it is better to sweep a paper wallet because of the reasons above too, which is for more security reasons.