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.
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.