Elastane (also called spandex or Lycra) is never used on its own in a cycling jersey — it's always blended with other fibers like polyester or nylon, typically making up 3-15% of the fabric. That small percentage makes a big difference, though. It gives the jersey stretch so it can hug your body without feeling restrictive, which is key for both comfort and aerodynamics on the bike.
Without elastane, a close-fitting jersey would be hard to pull on and even harder to move in. It also helps the jersey snap back into shape ride after ride, so your jersey doesn't end up baggy and sagging over time.
For road and performance riding, look for jerseys with at least 5% elastane for a secure, aerodynamic fit. For more relaxed or gravel jerseys, 3-5% is usually plenty — enough stretch for comfort without that skin-tight race feel.