Your questions don't appear to match the headline, but to try and help:
Why are gears used when there is automatic gear technology?
- Because automatic gears have more losses- so you don't get as much efficiency from your drive train.
- Because automatic gears can't read your mind and may not choose the best gear for what you want to do.
- Because the greater complexity leads to more maintenance or more breakdowns.
Why was this gear technology developed
That is just how power works. If you turn the drive cog more times for a number of output turns, you get more torque but less top speed. It's basic physics. Have a look at this howstuffworks page for a detailed explanation.
Does a bike with gears have any advantage
Absolutely! It is lighter, requires less maintenance, is potentially safer in an emergency situation, and in fact the only good thing about the automatic scooters is that they are very easy for beginners to learn on as they don't need to worry about gears, clutch etc.
An animation to explain Continuous Variable Gears (from Wikipedia):