example why did Nazi Germany bother to take Russian war prisoners?

Sad thing is, this was largely not the case in the beginning; Red Army POWs were taken, but of the estimated 3.2 million POWs taken between June 1941 and January 1942, 2.8 million died. Of the 5/5.5 million captured during the war, 3.5 million died.

they could have used the soldiers in the frontlines instead of guards

Guards were by-and-large not frontline soldiers; besides, the rear areas of the Wehrmacht were massive, and required extensive security anyways. Besides, Russian POWs provided forced labour, and large numbers of Russian and non-Russian POWs became Hilfswilliger or Hiwis, essentially auxiliaries that accompanied German units and provided labour and other functions. Some German units had as many as 2000 Hiwis attached to them. Many joined the Ost Bataillonen and Vlasov's Russian Liberation Organization.

Seems more cost effiecient to just kill your enemies instead of taking them to camps etc.

First of all, most prisoners are soldiers who have either given up fighting or have had their unit surrendered. Strictly speaking, they have lain down arms, and to simply kill them is a war crime. Moreover, prisoners provide Intel, they can give an idea of what the enemy army, it's equipment, it's morale, and it's organization is like. They can tell you what life is like behind the lines, and can provide various forms of labour, often extending to forced labour in WWI and II. Taking prisoners gives other soldiers incentive to surrender, which can be hugely beneficial when their morale is low, and typically when they (the enemy) are losing, and whole units can collapse; soldiers that know they have zero chance of survival will probably be more inclined to fight tooth-and-nail than those that have a way out (ie surrender). They also essentially serve as hostages; so long as your prisoners are treated well, their prisoners will be treated well.

Well you do it in the hope that the enemy will do the same. At the end of the war everyone exchanges prisoners as part of the peace process, however if you were to kill the prisoners you'd have a lot of long standing bad blood. Plus it's pretty grim stuff shooting at someone who is shooting at you, it's a whole other thing to kill someone who has surrendered, killing prisoners is pretty low and ordering a unit to do that could cause nasty morale problems.

Then there is the Geneva convention which governs expectations of belligerents in several ways including prisoners of war.

Regarding the question as far as Nazi Germany's treatment of Russian War Prisoners; the Nazis actually did have certain policies regarding Soviet POW's in particular. Over three million Soviet POW's were exterminated. I guess the bottom line is that the Nazis did eventually kill those who did not share their ideals, but the reason they used camps for their method of killing is because they seemed to be their most efficient option. Without death camps in particular, they would have millions of bodies just littering Europe, and they also had ethical issues regarding the executions among their own ranks because they had men who did not want to execute prisoners. (Fun Fact: Not everyone in the German Army was a Nazi)

Regarding the question about warfare in general, well, it really depends on the time period and prisoner type. With different moral principles and governing systems throughout time, there were different reasons to take different prisoners. Political Prisoners have been common throughout history. Julius Caesar was captured by pirates and ransomed, and then hundreds of years later King Richard was captured and ransomed, almost bankrupting his country in the process.

Some prisoners were taken in history just because the victors weren't didn't believe in slaughtering those who had surrendered. During the crusades, entire armies would surrender, and promptly be slaughtered. The crusades were a war of faith, and the moral duty of its participants was to eliminate the enemy. Fast forward to Modern Day, and there are Terrorists being held prisoner at places at Guantanamo Bay. Their crimes could get them sentences such as the death penalty, but prisoners like them have something in them that makes them more valuable alive, information.

Sorry if the answer was a bit vauge, I just felt I needed to jump around to give a basic understanding that conditions were different at different time periods.

TL:DR - While the situation regarding the acquisition of POW's may have changed over the past millennium, in general they have been for Money, Information, or Power. (And in the case of the Nazis, methodical execution.)