Gold or Fiat?
Why would you buy Gold instead of keeping your money in Fiat, or Microsoft stocks, or foreign currency, why would you buy an investment home, or put money into your superannuation, rather than rely on the country's default currency, the mighty dollar?
The answer of course is that the different assets have different properties, some go up while others go down. Moving funds from one to the other at the right time can net you a profit. Holding onto another past the time you should have sold can lose you the farm.
Currencies or assets (tokens) inside Ethereum's blockchains are not (directly) tied to the value of Ethereum. Some of them may actually be tied to real-world currencies or assets instead, and completely removed from the current value of cryptocurrencies altogether.
As Gabe points out in the comment, it's impossible to be completely separated from the value base of Ethereum in the same way that a house in the USA can't be separate from the value of it's base currency the USD, but at the same time your house's value is somewhat independent of the dollar and can appreciate and depreciate on it's own merits.
You see tokens can represent anything. I could have a token which is a deed to my car, and which people can use to rent my car, or buy it from me. They can represent securities, e-currencies, virtual kittens, revenue shares, houses, almost anything.
And just like the things they represent, some people want them more than they want the base currency, and some people want them less.