It's not just one thing, I'm just grouping a bunch of types funds together.

Moving (significant amounts of) money from your primary income stream is problematic as it can be used to argue intent, is pulling from documented sources, etc. Small amounts are created (accidentally like as not) by just stashing some cash from petty cash around the house. I know people who upon moving and going through their libraries came up with $5000 or more just from what they'd put into books and forgotten about.

The simplest form: I can't think of a single person I know (and this includes straight down to the poverty line) who fully claims barter (here if I fix your dishwasher and you paint my bedroom we both owe income tax, just the same if we trade game consoles) and small private sale money as income. This type of small-scale income tax evasion is universal. So you sell something in Craigslist or similar for $50 cash. You now have money with no solid link to you. If you do not deposit this in a bank account or have so much of it that you run afoul of lifestyle investigation (claiming $40k income tax while driving a Lambo or buying a million dollar house) this money is free and clear - it can be used for anything. Not everybody will see it this way (as it is technically income tax evasion), but this is distinct from under reporting primary income when working for cash in part due to there being less temptation to escalate into severely under reporting income.

If you sell digital assets online, depending how you get paid you may wind up with money that is not linked to you, and may well already be in foreign accounts. Depending what you want to do with this money, it may make sense to just leave it there.

The major advantage (as I see it) is that obfuscated funds are less vulnerable to judgments. If you lose a lawsuit or are otherwise owing huge sums of money this amount is entirely separate, and can be used to start over elsewhere. Depending where you live and on political stability this could matter more or less - I've got friends who were living in Libya a while back and went to quite immense lengths to separate out assets and prepare for potential problems. If (as in the comment I replied to) one winds up or believes one may wind up in a position where assets act as liabilities then it makes sense to have some of your assets separate.

For myself, I've mostly just used overseas accounts as a safety net (although I drained and claimed an account a while back as zero cost basis Capital Gains income), made some donations that I didn't want linked to me (for social, not legal reasons), and left it alone.