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.
Obviously employers may vary. I have hired dozens of people over the years and I have never:

* run a background check. I'm not sure it's even legal in Canada. We're not supposed to take a criminal record into account. Credit rating never made sense to me as a legitimate proxy for hire-ability.
* decided on someone else's behalf whether they could stand a commute or not - I hired someone once who had a 1.5 hr commute to downtown Toronto, he came to work for me without moving and had a 1 hour more pleasant commute in the opposite direction, to our office.
* covered anyone's travel expenses, whether to come and be interviewed or to come and work for me. We're just too small to afford it.

That said, I still like to see the address in the cover letter if not in the resume, and my children both provide resumes that include this. I think it's more a matter of respecting the rituals than it serving any practical purpose. After all, I can probably narrow down roughly where you live by looking at where you work now. Even if you see no good reason for including it, omitting it gives people an excuse to toss your resume aside; don't do that.