With all the altcoins on the market it's a bit hard to answer this.
coinmarketcap lists 876 altcoins today. When listed according to market value, the bottom 180 have an unknown supply. Well, that does it, you're hardly going to buy a percentage of an unknown quantity. But what about the other 600 or so?
Getting back to the question, why indeed would you invest in anything which has a high risk? Well, why would you buy a high-risk stock? The answers to these questions are the same. The usual answer is that one wants to get some profit and is willing to take some risk.
But that is not nearly always why people invest in a company. Sometimes we just like the idea. Sometimes we want to help a friend succeed. Sometimes we believe in people or ideas and we pitch in to do our part. The company may succeed or fail, so we try not to invest more than we can afford to lose. Ten years later we can say with a good conscience that we participated and if we are lucky we also made some money.
People often cite the volatility of Bitcoin as a major problem in terms of investing. But of course that is not a problem - it is merely a factor to be taken into account when investing: Do you have a bit of money that can lie around for a while and that you can afford to lose? In that case investing in Bitcoin is no worse than investing in any other high-risk startup (and most startups are). Similarly, if all you want is to try out electronic currencies, then buying some dollars worth of an altcoin is a good way to have fun with electronic currencies.
The main problem with the altcoins as a group is that a whole lot of them don't really seem to have a purpose and a lot of them have a really bad reputation. But there are also altcoins which have a very specific purpose and I want to talk about one of them (again). Beware that I am heavily involved with this one so this is hardly an unbiased assessment!
The Smileycoin (SMLY) is an altcoin specifically defined to be used inside a freely accessible educational system, the tutor-web. As students progress with their studies they get paid in SMLY. They can subsequently trade SMLY among themselves, buy coffee for SMLY, buy a number of other things -- or just sell off their SMLY at an exchange.
After months of deliberation and discussion, this summer the SMLY is being enhanced quite a bit. In particular, from August it will be modified so that each new mined block will contribute coins to a non-profit educational project called Education in a Suitcase. This means that EIAS will have an income which can be used to assist students in rural Africa get the best possible educational opportunities. To date EIAS has provided educational materials to several schools and set up tablets and servers for students to access the material. As the SMLY gets more backers this income may make a great deal of difference to what EIAS can do.
Another change to the SMLY will be that each mined block will also contain instructions to pay dividends to those who have acquired at least 25 M SMLY.
So here is an example of an altcoin which has a purpose and is being redesigned both to reward investors with formal dividends and to contribute to two great projects, the tutor-web and Education in a Suitcase.
Now, don't get me wrong: If you buy SMLY then you are taking a risk. You may lose your money in a few different ways. An exchange may go bust. The project may go bust. But if things go the way we want, your SMLY wallet will be paid dividends. And along the way you will have contributed to a really great project. Isn't that what life is all about?