Think of the front chainring's shifter as Uphill, Flats, Downhill and then it begins to make sense.
The middle chainring will gladly use all of the rear gears.
Using a rear derailer with no front derailer will quickly toss the chain off the single front chainring.
Yet, until that happens, the system provides enjoyable, fast shifting. Of course, you can use a front derailer that does nothing but hold the chain on. For 11-34 combination, 9 speeds and expense is unnecessary when it is available in 7 if you use a $2 spacer and a $23 indexing lever shifter to work a Deore rear derailer.
Whatever rear-only system you choose, make certain that you use a lever shifter (use a bar end shifter if you get that 9 speed), because you must cross over every gear very slowly if you have a packet shifter.
A Nexus 8 provides the same gearing as that hyper-expensive 9, and they are available pre-built into a 700c wheel for just over $250. The road version is called "premium" and is lighter weight. Shifting is extraordinarily fast. Range is like a road double with a road cassette. You get any gear at any time, under every condition. Revo and packet shift are both available. Weight is quite a lot, but not more than an entire Ultegra derailer system plus a rear freehub. Sachs makes a more durable 7 with the same range (as Nexus 8) for MTB application. Rohloff $1000 to $1500 is a similar product with far more range and extremely fast shifting. It is a heavy MTB or tandem product and requires a large size front chainring if it will be used at roadie speeds. Any IG hub has a break-in period of 300 miles before they loosen up and speed up. After that, it is about 22000 miles until maintenance. It will outlast its rim. Despite a possible addition of a bit of weight (in case of the Swiss/German-made products), the IG hub can accelerate so fast that it pays its way. The Nexus 8 is also known for favoring the high gears with less drag than a derailer system. And, it just makes sense.