Question: Mail era is gone. Now we contact either by email or phone, so why is it still necessary? 

I have read it is to let the employer know where you live, but why is it necessary? If I'm applying for a job which is far from home is because I know I will have to relocate, isn't it obvious?

Answer: 
In general, when you apply for a job (or rent an apartment, apply for a credit card, etc) folks want to do a background check on you to make sure you're not a felon, or a credit risk, or a thief, or someone who for whatever reason won't qualify for the role because your personal profile represents unnecessary risks to the business that outweigh whatever skills you bring to the table.

Your credit report, criminal history, and mode of living can all be tied to your current and previous addresses, and background screening companies use this information to verify you're not an employment risk.

According to career coach Terri Lee Ryan, author of [Should You List Your Home Address on Your Resume?](http://www.chicagonow.com/get-employed/2011/01/should-you-list-your-home-address-on-your-resume/), employers may use a pre-screening process where any resumes that don't include this information are tossed out. With so many applicants, screeners may use such techniques to narrow the pile to an array of candidates more manageable.
While there is e-mail and phone as ways to contact someone, how do I know how local someone is when they apply for a job? If I have 100+ candidates that all meet my initial requirements, wouldn't it make sense to interview the local people first? If someone has to move this can cost in both money and time in some cases. The key point here is that if I have a pile of resumes, how does the company distinguish who should get an interview?

There can also be that odd personal touch for some people, like an "I went to school near there," or "Oh that's a nice neighborhood," as there can be things inferred from the address,e.g. if someone lived in Beverly Hills or Compton for a couple of parts of California that one could contrast.