Russia media recently ran a story about how pilots of the new Russian T-50 (similar to the F-22 and still in development) would be issued g-suits so that, if necessary, they could eject from their aircraft and not suffer the deadly effects of altitudes that high (where your blood starts to boil). The tight fitting pressure suits protect pilots from this sort of thing, which starts to happen at about 20,000 meters (62,000 feet). The T-50 is designed to fly as high as 23,000 meters (75,000 feet).
Pressure suits also protect against the effects of very high gravity (as experienced when sharply turning a speeding aircraft) and are also called G suits. Actually there are two different types of special flight suit that have similar characteristics. These suits have been around since World War II but are rarely used in fighters, as the suits are too restrictive. The main reason for G suits is to deal with G (gravity) forces, not high altitudes. For over half a century, aircraft have been capable to executing maneuvers (usually sharp turns) while moving at high speeds, that create a gravitational force (G-force) that causes the pilot to black out. If a pilot is properly equipped, with special flight suits that use small liquid or air filled bladders to help prevent blood from rushing from the brain, and causing a blackout, during high G force maneuvers, a G-force nine times normal gravity ("9 Gs") can be tolerated. But even with the G suit, pilots have to use their abdominal muscles and a deep breath to avoid blackout.
Fighters have pressurized cockpits and if a pilot were flying higher than 20,000 meters and the cockpit lost pressure for any reason, the pilot would be in big trouble without a pressure suit. But pilots don’t expect to operate at those altitudes and in Western air forces fighter pilots are usually forbidden to fly higher than 17,000 meters (51,000 feet). For one thing, above that altitude the thin air reduces the power of jet engines (which need air containing oxygen to burn the fuel to make the engine, and the aircraft, go).