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: 
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.
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.