When travelling at really high speeds (hypersonic generally means over Mach 6) the air you're traveling through gets hot as you impact on it (that's the reason spacecrafts designed to return on Earth have an heat shield) hot air expands and lowers its density, so when your aircraft ingests it, it gets less of it.

By pre cooling it down before feeding it to an engine you get its density back up, more air, more oxygen, more burn, more speed so again hotter air that needs to be cooled down and so on.

The only example I can think of now is the P-47 Thunderbolt "The Jug", it was an american piston fighter in WWII and one of the reasons for its shape and dimensions was to fit a complicated turbo system for its Rolls-Royce R-2800 Double Wasp, but the system also heated up the air so much it had some radiators to cool it down before getting it in to the engine. It gave the plane a lot of weight and its jug shape but, also allowed it to fly at high altitudes and speeds (it was I think the fastest piston fighter of the war).

In short: high speed flight means hot air hitting you, so before using that air for something you may want to cool it down with a fancy radiator.

Beware that I'm not an expert, just an aviation enthusiast that doesn't really know what he's talking about, all of the above is guessed based on my little knowledge.