Whey and casein are both derived from milk - whey is the liquid portion and casein is the solid curd. Despite their common origin, they behave very differently in your body, making each optimal for specific situations.
Whey is fast-digesting: it raises blood amino acid levels within 60-90 minutes of consumption, peaks sharply, then returns to baseline. This makes whey ideal post-workout when a rapid influx of amino acids can accelerate muscle protein synthesis. The fast peak is also advantageous first thing in the morning when fasting has depleted amino acid levels. Casein digests over 5-7 hours, maintaining elevated blood amino acid levels for a sustained period. This "anti-catabolic" effect makes casein ideal before sleep when your body repairs tissue during an 8-hour fasting period.
Do you need both? Not necessarily. Most people get adequate results using whey alone, consumed consistently throughout the day. However, if you're serious about maximising muscle growth and can afford the additional supplement cost, using whey post-workout and casein before bed covers all timing windows. A more practical (and cost-effective) alternative to casein powder is cottage cheese or Greek yoghurt before bed - both are primarily casein protein from whole food sources at far lower cost than supplement form. The difference between optimal timing and good timing is small; consistent protein intake throughout the day matters far more than timing nuances.