why once a seed(created originally in BRD) is imported into a new Electrum wallet, a new seed should be generated ?
You are confusing by yourself.
Importing a wallet seed or a wallet private key into Electrum or any wallet software, means you are using a same wallet by importing it to a new wallet software or a same wallet software on a new device.
Importing never means you will create a new wallet, this is what you are confusing.
Creating a new wallet is recommended as you don't have to spend extra cost to create a new wallet, so why do you not do this if it can give you safer and better practice to store your bitcoin.