You could in a way that in those HFT hedge funds, you ARE building something. A highly complex trading system that with high probability gives your investors an annual return higher than inflation. That's at least how I look at it if I need to find meaning in my job.

Anyway, those firms mentioned are in the algorithmic trading space. People tend to throw the word HFT around quite loosely so I hope to clear two things.

HFT vs Algorithmic trading:
High Frequency Trading (HFT) is actually a subset of algorithmic trading. Algorithmic trading simply means any sort of trading that is automated with a computer eliminating the need of a real person monitoring prices live and trading on them. Again, this should be seen as a type of trading where the emotional and human aspect of it is removed and replaced in its place a calculative and numerical approach of how to trade. No assumption is made on whether the need of a human trader is beneficial or detrimental

HFT is a subset of this style of trading where both the time to generate trading signals and the time between entering and exiting a trade is within a second. Obviously, this by default necessitates the use of a computer.

For an example of algorithmic trading which is not HFT, consider the following. A system which looks at a whole bunch of data, say two days worth of 5 min data of three interlinked assets or ~1500 data points, does complicated calculations on them, say an inference of their probability distribution, and then decides on a position which presumably is trend following meaning that it's exit would be in a weekly time frame. Clearly, there isn't any high frequency component by way of trading but the need of a computer is obvious.

Hedge funds vs Market Making:
I know that Getco is a market maker and not a hedge fund. Hedge funds takes investors money and make speculative decisions on the direction to which the market is moving. A market maker collates a whole bunch of prices, from banks and dealers, and puts out their own prices at which other speculators, this would be hedge funds, could buy or sells certain assets.

Of course, in an age of high-tech finance where very smart people are finding innovative ways to make money, there is some overlap of strategies hedge funds and market makers can use. That will be a conversation for another day.

Cheers,
Donny