The job of brake hose is to give a channel by which the fluid can act. When the brake pedal is pushed the pressure is transferred into the brake fluid which transfers the pressure tot he brake pad.
The brake hose must no leak, it must be flexible and it should not bulge.
Obviously if a brake hose leaks the pressure from the brake pedal is wasted by forcing the fluid out of the braking system instead of pushing the brake pad against the brake discs. If the leak is bad enough it will lead to all of the brake fluid escaping from the brake system resulting in no brakes.
The brake hoses must be flexible. The brake caliper moves up and down with the wheel in relation tot he body of the car. The brake hoses must be able to sustain this constant movement indefinitely while still performing its job.
Brake hoses must not bulge. As brake fluid gets hot in can make the brake lines more flexible meaning that instead of the pedal force being transmitted to the piston in the brake caliper the force is used to bulge the brake line. If the brake line is old enough and/or abused enough this can lead to a soft brake pedal.
Performance brake hoses from firms like HEL are designed never to bulge and to last the lifetime of the vehicle.
While performance brake lines are not as critical as performance brake pads they are another step in improving the capabilities of your cars brake system.