They have low self esteem and by picking fun at others makes them feel bigger and more inferior. In reality they are just bullies and need a hug. I know it hurts but ignore their comments. They should eventually stop. They are doing it because they know it hurts you.
It's a byproduct of evolution. All social species establish a "pecking order", by imposing ones will on those around them an animal moves up in the hierarchy of the pack/herd/tribe, in humans this can also be done verbally. It is completely unnecessary however because it has become financial strength that determines our pecking order.
No purpose at all. They're just mean and some enjoy being cruel, and watching others suffer. The best thing is to not pay them back in the same way. Don't stoop to their level, keep your dignity and they may end up respecting you.
It is all about control. To control someone, you have to be in a leadership position. That position has to be given to you by some kind of authority or you have to convince people that you deserve it. Since people will not so easily give up their freedom it becomes necessary to use force to extract their obedience and compliance. The problem starts when violence is not enough to maintain control which leads to a revolution. Some people need this level of control just to function. Some beat their pets or their family members or worse kidnap or murder or torture someone. As a society, we have created laws to protect unsuspecting individuals from these control freaks. But since you can't have a cop in every corner, this issue continually re-occurs in our society. Obviously, a control freak will go after the easiest prey. You should never allow such people to get the upper hand in any situation.
The answer to this can be found in the Bible. At 2 Timothy 3:1-5 it states, "But know this, that in the last days critical times hard to deal with will be here. For men will be lovers of themselves, lovers of money, self-assuming, haughty, blasphemers, disobedient to parents, unthankful, disloyal, having no natural affection, not open to any agreement, slanderers, without self-control, fierce, without love of goodness, betrayers, headstrong, puffed up with pride, lovers of pleasures rather than lovers of God, having a form of godly devotion but proving false to its power; and from these turn away." We see this everyday; it reads as a newspaper headline.
However, God promises at Psalms 37:9,10, "And just a little while longer, and the wicked one will be no more; and you will certainly give attention to his place, and he will not be. But the meek ones themselves will possess the earth, and they will indeed find their exquisite delight in the abundance of peace."
God sees injustices and will intervene soon. I hope this answers your question.