Server architecture: more cpu cores, more cache (resulting in better performance per clock period), detection of RAM errors (1-bit and 2-bit) and correction for those (1-bit), built for non-stop operation.
ECC arch. is worth the money wherever correctness of the computed results or integrity and stability of running programs is imperative - scientific and engineering calculations, database systems handling precious information (money), etc.
In practice, it seems ECC in a personal computer is not necessary for most people, but in 2009, a study of DRAM errors in datacenters, ECC was found to be very benefitial:
"Across the entire fleet, 8.2% of all DIMMs are affected by correctable errors and an average DIMM experiences nearly 4000 correctable errors per year"