Fighting is the physical embodiment of conflict. Two parties believe they are right, so they fight to prove it. Sometimes this is problematic, and people get hurt in defense of a side. In other cases, fights can be beneficial as parties unite to reach a compromise. For me, at least in the case of boxing or professional wrestling, fighting is pure entertainment. This is still two opposing parties working to solve a difference. In short, fighting is one way of resolving differences.
We often think of fighting in a negative light, focusing on the pain and destructive nature of fights. There can be a positive note too, though. Fighting is often indicative of passion--both positive and negative. In most cases, if a person (or group) is willing to put up a fight on an issue, this demonstrates a desire to change another’s mind. I believe this brings notice to causes which deserve the attention. More passion (on either side of the argument) highlights the causes more important to society.
Problems arise when this cause might not actually be fight-worthy. Particularly pugilistic people may create arguments just for the sake of having a fight. This antagonism is hardly beneficial, especially when the recipient is someone who avoids conflict and caves into the other argument. Unfortunately, this is the kind of fighting often seen online, where someone can hide behind anonymity and cause a ruckus. This is problematic as fights can escalate into something closer resembling an attack.
Just as fighting can be destructive, though, it can also be constructive. Fighting (or arguing) more on the personal level, between friends or family members for example, can make both sides stronger. A person has the chance to make their case and feel heard. Although it may be frustrating or downright hurtful at the time, persevering through fights can lead to compromise.
Essentially, fighting over a worthy cause is a good thing (as long as either party does not get too seriously hurt, of course). It shows passion, which focuses on good causes. At the public level of fighting, say in a political setting, intelligent debaters will use smart arguments to lead to the right conclusions (hopefully). At the micro level, meanwhile, a fight among friends or partners can lead to a stronger, healthier relationship.