the only mythical item that truly benefits the team as a whole, the Runic Bulwark is build on almost every match. at a cost of 3200g, it gives you health, armor, magic resist, and an aura that bestows part of said statuses on your allies. Whether on higher or lower elo, your team should build at least one of this item. The question is: who should build it?
Junglers: while building a Bulwark denies you of that damage item you so crave, remember that it's not your job to deal damage, it's your carry's. Your main role as a jungler (especially a tanky one) is to initiate a fight, or isolate one target so your team are able to collapse on said target. building an emblem of valor which would eventually build into an aegis and a bulwark helps early-mid jungling because it gives a regen aura and armor for yourself. for early teamfights, you should have an Aegis of the Legion ready to help with those engagements. by the time you have the Runic Bulwark, when you initiate or isolate the enemy team, your teammates would have the aura when they got there.
Supports: most solo queue junglers who refuses to build Runic Bulwark would force the support to build the item instead. while it is true Emblem of Valor would give the AD carry the ability to stay on lane longer, there are other, better items for supports to get that benefit their team more such as Shurelya's Reverie, Ruby Sightstone, or Locket of the Iron Solari. with the exception of inherently tanky supports (leona, Alistar, Taric, Blitz, Thresh), champions that are played on this role are usually squishy. even if they build Runic Bulwark, they are not supposed to be in the front of the team, or they would be targeted, killed, and lose their ability to support. unless your support is inherently tanky or has the ability to initiate fights, i suggest bypassing bulwark for other items that give the same benefit and then some.
Top Laners: with the rise of some assassin-oriented top laners, it's rare to see a Bulwark build on a top laner. however, some top laners that are really tanky or has the ability to initiate a fight (Malphite, Olaf, Shen) still would build a Bulwark from time to time. however, even they would more often rush a Warmog's Armor or a Sunfire Cape.
Bottom line is, the Runic Bulwark is a versatile item that could be build on three of five positions in the current meta. however, this item is best build on someone who is able to initiate a fight, to get the bonuses to the team faster and more efficiently. this usually translates to a jungler or a tanky support. building this item on said champion would guarantee that the aura is wherever a teamfight is about to occur.