I am still new to Bitcoin mining and there are many things need to learn, my question is, why does Bitcoin mining use GPU? why can't I use a normal PC to mine bitcoins? what are the reasons? can anyone explain me?
CPU are too slow, and do not provide enough compute capacity for crypto-mining. On a Intel Core i7, you have 4 Cores, and 8 Threads (2 logical cores for each physical core), when you can have up to 392+ cores on a GPU. So for the same energy used, you will mine 400x more with a GPU than with a CPU.
CPU is smart and fast, it has to be ready for any command and quickly make a decision, those commands differ from one another, in another hand GPU is dumb, it has one job, that is to take care of Pixels, a repetitive job which fits perfectly with BTC mining, the poor CPU is always busy for these stuff.
GPU is a graphics processing unit, which is mainly used for 3D applications. When the GPU transforms lighting effects to 3D, it performs mathematical tasks. Blockchain, the BTC technology is basicallyt a mathemetical task. CPU can also process this task, but it is very slow because unlike GPU, it does not havr thousands of core.
You shouldn't really focus on mining if you don't have a high end PC, not only that, but you also need ASIC, which is a special hardware used to mine so the bills at the end won't kill you. As other people stated you can start looking at other cryptocurrencies, they are starting to bloom and a lot of them are appearing, so google some other types and learn.
It's probably better to invest into newer upcoming crypto currency like ethereum, rather than buying GPUs and mining them atm ( BTC or ETH ), big companies are buying 1,000s of ASICS miners to mine BTC, its almost non profitable to mine BTC now, unless you have free electricity
CPU is intelligent and quick, it has to be prepared for any control and easily create up your mind, those instructions vary from one another, in another side GPU is foolish, it has one job, that is to deal with Pixels.