A pricier speedlight usually buys you features, compatibility, and reliability rather than just “more light.” Common advantages include:

• TTL auto flash metering (e-TTL/i-TTL/P-TTL) for easier automatic exposure
• Better camera/flash integration, including control from camera menus on supported bodies
• Motorized zoom head, and often more flexible tilt/swivel movement
• More power and sometimes faster recycle times
• Better wireless options beyond simple optical slave modes
• Stronger build quality (especially the hot shoe), better UI/LCD, and sometimes weather sealing
• Better consistency, service/support, and long-term compatibility

OEM flashes also tend to work more reliably with current and future camera features because the camera maker knows the communication protocol; cheaper third-party units may reverse-engineer it and can lag in compatibility.

That said, a cheap manual flash can still be excellent for learning lighting or for off-camera manual use. If you mainly want low cost and are happy to set power manually, inexpensive flashes are often great value. If you want automation, dependable integration, and pro-oriented features, the more expensive flash is where the extra money goes.