Reha redback has a couple key features that make it more available/scalable than other riots, as well as some good unit matchups.
-low profile, spherical hitvolume, weapon shoots from above top, high turn and turret turn rate.
These physical characteristics make it really easy for redback to avoid blocking allied redbacks line of fire, allowing multiple redbacks in a mass losing less damage output than other units.
-hitscan laser weapon
While redback has no AOE, this sort of riot has strong single target damage instead of crowd control, like mace instead of reaver/ripper, and careful micro can allow this unit to deal full damage while staying out of range of units it only slightly outranges.
High level players especially can abuse terrain to get more favorable engagements, like using terrain to close the distance with a defensive turret that has been placed properly for a ramp but not the sheer cliffface.
There's other features that promote the building of redback, like:
-good cheep dps in a factory missing it. Other riot has low dps, raider is scout that dies too quick to be general combat.
So spider players are more likely to spam the riot than other units, which makes people more familiar with the unit in the game.
Which brings attention to redback versus other units.
-just enough HP to survive a stinger shot
-almost no overkill damage against glaive
-outranges reaver and some other riots, can gain ground/terrain against skirmishers with on-a-dime turning
And a potent spam unit. I've done this strategy before: mass riot. Fake out enemy skirmishers with good movement or peeling off a couple, and run all your riots into enemy main/factory. Turrets and factory production do not have enough stopping power against 9 riots like redback, but redback has the ability to ambush poorly paced turrets or bypass them all together with spider legs, which can make this strategy even more potent.
This is essentially my case for why redback is a good to spam unit under spider factory.