The Crate Potion increases the chance to find a crate while fishing. It can be very useful when grinding for crate items such as the Hardy Saddle.
Crates can be very useful both before and after Hard Mode is enabled. They can provide a player with a relatively stable supply of ores, with the ability to provide all six of the first Hard Mode ores once Hard Mode is enabled. They also have a chance to award other rare items depending on the type of crate.
In order of which is the most likely to give Hard Mode ores when opened: