The value proposition for the investor in an ICO is that if the currency sees adoption for real transactions, its value will rise. The value of a currency is fundamentally determined by supply (total units in circulation) and demand (what you can buy with it). If you start being able to buy more things with it, and supply remains constant, the value of the currency rises. This is why the Fed is constantly injecting money into the economy: economic growth means that you can buy more things with a dollar, which means that if the money supply remained constant (as it did under the gold standard of the late 1800s), the value of the dollar would continually increase and prices would continually decrease.

With an ICO, you lock up some of that money supply in speculators' wallets, in exchange for the initial capital infusion. As the normal use-value price of the currency rises, they will sell and take their profits, injecting money back into the economy.

The wrinkles right now are that a.) nobody really knows how to value a currency with few-to-zero users and b.) a lot of these projects didn't deliver. That meant that a number of people lost their shirt in 2017 (actually, I suspect that number is less than commonly reported: I think most ICO investors were early Bitcoin and Ethereum hodlers who had already cashed out their initial investment into the bubble and then had some play money to diversify). But if you can pick winners now, when all the initial hype has disappeared, you stand to make a fair bit as people start using these platforms for real commerce.