Answer
Springtails can help consume mould spores and small organic debris in moisture-retentive setups. They are optional and are most useful where the substrate stays damp enough for them to survive.
Best use
Moisture-tolerant terrestrial, fossorial and planted setups are the obvious fit. Very dry enclosures may not support a useful springtail population.
What they do not replace
Still remove large boluses, dead feeders and visible waste. Springtails are a cleanup aid, not a substitute for enclosure maintenance.
Do not overwater for them
The tarantula's moisture requirement comes first. If springtails disappear in a dry-adapted enclosure, do not make the enclosure wetter simply to keep the cleanup crew alive.
Springtails or mites?
Springtails move quickly and often jump when disturbed. If you see unknown arthropods clustered directly on the tarantula or around the mouthparts, inspect more carefully rather than assuming they are harmless springtails.