They both have their purposes. Yes the archer is stronger and has longer range but it is slower. Archers are great as garrisoned units, especially with the promotion that boosts them in cities and districts. If you don't need to move your archers a lot, they are great as defensive units. But as you pointed out, the Saka horse archer has a movement of 4, making a very fast unit. So with the "build 1, get 1 free" ability, you can spam lots of saka horse achers. Individually, they are weaker than a single archer unit, but you have more of them and they can move further. So you can build a larger force that can strike fast and move fast. You are relying on numbers. 4 saka horse archers all attacking a single unit at once will be stronger than a single archer attacking the unit.