What is the role of miners in Blockchain? What is the need of validating and authorizing a BTC transaction.
Say, for eg, A have 5 BTC in its wallet and he wants to send 3 BTC to B. What is the need to validate such a transaction?
And in case A wants to send 7 BTC to B, he simply cannot because he doesnt have the sufficient balance.
What is the need to validate then.
Why cant it be possible that the transactions are automatically added as a next block on the blockcahin on all the nodes?
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It is necessary to validate all transactions since it is possible otherwise for A to just broadcast a transaction sending 7 BTC to B even if he does not have enough. A could easily write his own program to do so.

Validating all transactions and the entire blockchain is necessary for Bitcoin to have any value and to ensure consensus rules are followed.