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: 
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.
Commuting to work can be a major factor in selecting a job. Either in larger cities because of traffic or in more rural areas based on distance. 

Depending on your current address, this may not be a factor so there's no reason to mention it. If I thought someone would have a long commute, I would want to make sure they were aware of it and try to get any past history on willingness to have a long commute. 

Many people will say they are willing to live with a long commute, but may be more likely to take a similar job if it is closer. In some major cities, commuting can be very expensive if you're paying for parking, train tickets or even gas.

Your address can be used to have more information about your prior work experience and probably to do a background check before hiring. Some names are common, so having a current address can make it easier to check the right John Smith.