The most important reward unlocked through the Forge are the Defense and Offence cards for each faction.
Battles take place in the battleground corresponding to the player who goes second (the ‘defender’), who benefits from the ‘home field advantage’ of choosing one extra starting card from among the defensive cards it has unlocked for his faction.
The player who goes first (the ‘attacker’), on the other hand, chooses when to attack. This is represented by having the chance to choose an ‘environment’ effect from among those he has unlocked in the Forge track of the defender’s faction. Eg. progressing through the Orks track is necessary to unlock all environment options when playing against Orks and going first. This means that, regardless of which army players decide to play with, to have access to all Offence options in all scenarios they will need to complete the Forge track for all factions, which they can only do by collecting cards for each faction.
All Offence effects will affect both the attacker and the defender. However, choosing the effect that presents the most hindrance to your opponent's playstyle and the most advantage to your own deck's strategy will be key to gain an edge in combat, so the more options you have the better!