What a tune-up is not
It is not a cure for a specific fault. If you have a misfire, a rough idle, or a check engine light, that needs diagnosis — a tune-up performed hopefully at a car with a real fault is money spent without a plan.
When it genuinely helps
Worn plugs cause misfires under load, poor fuel economy, and hard starting. If you are at or past the interval and noticing any of those, replacing them is straightforward maintenance with a real payoff.