Your collateral top-up, your trade, or the price update your position depends on shouldn't be at the mercy of a single collator.
Without protection, a malicious collator can withhold a block to make the next collator miss their turn, then use that gap to censor transactions. That's enough to get a position liquidated, a trade sandwiched, or a price feed held back.
Polkadot hardened parachains against this with multi-slot collation, giving collators a longer window to recover from it.