What is the concept of tip?

I have always wondered why does the system of tip exist and what exactly does it mean?
The present practice is to leave around 10% of the bill in restaurants and about 50-100 rupees to waiters or doorman in hotels. But what exactly is the need for paying?
What is the need for tip? We already pay for the service a waiter or anyone does in the bill. No restaurant or hotel is cheap these days. So why do we need to pay when the guy is doing only the regular job for which he is already getting paid?
For myself, I only give tips when service is not ok but really good. Otherwise I dont feel necessary at all to leave even a single penny.
And I am not writing this post or even do this to save money. But I dont like seeing how the practice of tips is being misused. Especially here in hostel when everyone even after bakwaas service people come for baksheesh.