Thanks for clearing the doubt, because I came across an article where they mentioned you can also use the private keys to create a watch-only address, probably it was available in old version or never had such a feature?
There are some misleading articles online, that is why forums like Bitcointalk are very important to get valuable information. How private key is designed, it is designed to fully recover your wallet. The private key itself can generate signature anytime you want to spend, but public keys and addresses can not generate the digital signature, which means they can not be used to spend (but only used as watch-only). So, addresses and public keys can not generate digital signature, in which they can not be used to spend but only as watch-only wallet. In this regard, the private key is very important, it is able to generate the public key and the address will be generated, while generating digital signature to unlock bitcoin on blockchain and spend it which can not make it be used as watch-only on wallets but to spend. In this regard, anyone that has the private key generated by a particular wallet can be use to unlock and spend the bitcoin the private keys controls on blockchain. 
