You want ECC in your desktop if you cannot afford your data to become (silently) corrupted. A single bit-flip in a JPEG image can totally ruin it. And you won't notice without opening it, because the thumbnail is unaffected. When you finally notice, the corruption has possibly spread to backups already.

For servers that is especially important, because data will reside cached in memory for days or even weeks. Also ECC is often the last line of defense against Rowhammer.

FIT (failures in time, or failures per billion hours) are about 20-30 per DRAM (18 per DIMM in the study) at a fault level, which may not rise to an error condition. ECC corrects many faults, if present, which don't become errors.

So expect a (4GB in this study) DIMM to have a fault once per 200 years on average. Of course, this is server-class hardware running ideal conditions.

This is, notably, in stark contrast to Google's results (https://research.google.com/pubs/archive/35162.pdf&) of about 20,000-70,000 FIT per Mbit, but closer reading suggests that there were a fairly large number of high-fault DIMMs affecting the mean and Google historically bought the cheapest hardware available. Even at this fault rate an average DIMM wouldn't fail in any given month, for example.

Personal gaming, browsing and light document work PCs do not need ECC. Video/photo editing/development PCs might be on the threshold for not wanting to produce or store corrupted work.

Multiply the risk out for any organization with many people doing the same kind of work where corruption on a single workstation could cause annoying trouble for everyone else.

tl;dr: one bit error in 4GB every 72 hours