Javelin throwers completely take Rus horse archers out of the equation, and with the permanent camel rider debuff against their cavalry, you turn a cavalry civilization into a mass infantry one — which doesn't feel good for Rus.

Javelin throwers also give you an opening to block a fast castle strat from Rus. The Byzantines have really good timing with a Cheirosiphon build into a javelin thrower push; you can catch them as they're building their landmark.

Against Rus, you can play greedy as Byzantines because you have such great counter units. Placing the Grand Winery between two berry bushes can boost your javelin thrower production significantly.

Once you get the second batch of javelin throwers, they become really threatening to archers because you can one-shot them.

Limitanei's damage reduction is really effective against Streltsy's spread damage over time.