Crates are caught while fishing and open into tiered loot — ores, accessories and biome-specific gear. Higher-tier crates in Hardmode drop Hardmode bars.
Higher-tier crates hold better loot. In Hardmode, crates such as the Golden and Titanium Crate can yield Hardmode ores and rare accessories.
Yes — crates opened in Hardmode roll Hardmode bars, making crate farming an early alternative to mining.