This allows them to attack much larger prey like spider monkeys and chickens, which they liquify on the inside with their venom, a kind of external digestive process.
After the liquefaction is well under way with the venom paralyzed prey, they suck out the nutrient rich liquid, often sharing the hapless victim with sister tarantulas from the same nest.
Thankfully, blue giant tarantulas are entirely a figment of my imagination.