The reason is that since i carry my mobile always with me, i do not want seed to be stored in my phone, A watch only wallet will help me see my funds and in case of a phone lost, my bitcoins will not be at risk.
Best way would be to connect hardware wallet with your electrum wallet, so nothing will be saved on your desktop or on your mobile device.
With watch only wallets you can't send coins and make transactions, but when you connect hardware wallet you can do everything and monitor your addresses.