Is it a good idea to contact professors before applying to grad schools? I've seen some universities encouraging students to contact with the faculty before making applications. For example,  this one. On the other hand I've heard some people saying that they are waste of times because professors receive several such e-mails every year and they don't even have time to look at them.

So, is it good idea to make contanct professors before making the application? Can it increase my chances of acceptance? Can it ever backfire me?
This really only works as a strategy if you have something truly meaningful to say. If you have to force yourself to do it and struggle to come up with something to say in your email, you're not helping yourself. Faculty get tons of email all the time and every year - as you say - get emails from prospective students, many of whom will not be admitted and in whom they invest no effort. So, if you genuinely think you're a good fit for a program/lab and have solid questions in mind to ask the faculty/PI of that lab that will communicate your quality and fit, this may be viable. But, I would definitely not recommend it as a general strategy.