Choosing a particular subject and its influence on our real life

That should certainly help but in your career only as far our overall knowledge is concerned we must look beyond our expertise for sure. For instance I'm an expert in welding technology now who would want me to read if I keep writing about electrodes and flux?

There is no option to choose a particular subject for higher studies. You have to choose. You can't study medicine, engineering and law simultaneously. What matters is not just choosing a subject. What matters is choosing the subject that is most appropriate and comensurate with an individual's interest and aptitude.

We have to choose a particular subject to gain knowledge and a degree to sustain and make a living out of it. But once the formal education is over, we don't stop learning, in fact what we choose as a discipline as in academics definitely helps us broaden our knowledge, but we gain even more through our experiences, by interacting with people, through trial and error. So both formal education (on a particular subject during our formative years) and post completion of education (through experiences) we learn and hence we need both.

Yes. Choosing a subject of your liking will certainly influence life in a positive manner. Though I'm a science graduate, I chose Advertising and web designing. This made me more focused on the subject and also made me a professional. I could see a purpose and meaning to life. I  lead a contented life and a creative satisfaction. Several booklets were designed by me are the curriculum for several schools in my state AP.

There is no doubt theoretically if you are sound than you are perfect one has to be practically sound too to be perfect in anything.

Normally we chose a subject like medicine or engineering, etc. that has more career prospects in the physical world. Better career means better earnings and better living. Life is different from living. Living is a materialistic aspect; but life is emotional. No subject will teach you real life; you have to experience it from reality and improve it yourself. 

Now a days choosing particular will not influence on our life,why because if you study mechanical engineer you will not work on that you will work as a software engineer.