Weak recordings usually share the same handful of problems — thin bass, boxy mids, or dull highs — all of which map directly onto this tool's four bands.
Before boosting anything, identify what's actually wrong: thin means low band needs a boost, boxy means low-mid needs a cut, dull means high band needs a boost. Fixing the right band matters more than how much you move any single slider.
EQ reshapes the balance between frequencies already present in a recording — it can't recover detail that was never captured, like audio recorded through a very low-quality microphone. For genuinely damaged audio, EQ helps but won't work miracles.