You can quickly special summon multiple elemental heroes at a time. Cards like HERO's Bond lets you swarm the field, while A Hero Emerges can be used to special summon strong higher leveled ones. Since most players often only set 2 spell/trap cards, a simple normal summon with a fusion summon is enough to clear their entire back row. This can lead to an easy OTK if you also have a De-Fusion in your hand.
Destroys Spell Cards / Destroys Trap Cards
I just tried to use this card but it doesn’t activate for me. I have like 3 EHero and the opponent have 1 set card in the Spell and Trap card zones
The rules say to destroy spells and traps equal to the number of elemental heroes. Note that it doesn't say "up to" the number of heroes. Meaning that if your opponent has like 2 cards facedown and you have 3 heroes you won't be able to use righteous justice unless you sacrifice one of your own spells and traps or wait until your opponent has 3 on their side of the field.
Also should I still keep it in my deck considering that it destroys “up to”
Summon Blazeman, activate 1st effect - get Poly from deck and fuse whatever 2 E-herous you got in hand, activate 2nd effect - send Neos to GY to increase Blazeman attack's to 2500, activate R-Righteous justice to clear backrow, gg