Rusted suspension bolts and exhaust studs will snap if you try to force them. Standard lubricants like WD-40 or engine oil are too thick to get into the tiny gaps between rusted threads. Penetrating oil is specifically blended with a very low viscosity and low surface tension. This allows it to creep deep into microscopic clearances by capillary action, dissolving rust and leaving a thin lubricating film.
For best results, spray the seized bolt and let it sit for at least 15 to 30 minutes before turning. Tapping the bolt head with a hammer helps create vibrations that draw the oil further into the threads. If the fastener is severely rusted, apply the oil several times over a few hours. Keep it away from rubber bushings and brake rotors, as the solvents can swell rubber and contaminate brake pads.