Based on the design of the circuit and on the description, what is its purpose?
It says that it transfers the state of D to Q. This means it can store one bit.
But what is the purpose of a D Flip-Flop when I can just put a lever on a block? This also can store one bit.
21 Answer
Saying that you can replace D Flip-Flops with a lever on a block is a bit naive. The power of D Flip-Flops becomes apparent when you use many of them, and connect them all to the same clock. You can't possibly toggle half a dozen levers all at the exact same time, but you can effectively do that with D Flip-Flops.
D Flip-Flops form the basis of shift registers, which in turn are a key component in parallel to serial and serial to parallel converters. These probably aren't very useful in Minecraft; using only two redstone lines (clock and data) instead of the parallel bus width doesn't make up for the slow speed of the serial line compared to a parallel bus. At best, we might be able to clock a serial line twice as fast as a parallel line, which is not a high enough speed increase.
However, D Flip-Flops are also commonly used as a buffer for parallel bus inputting to a complicated circuit (although the simpler D Latch is often more appropriate). A buffer is just a bunch of D Flip-Flops all connected to a parallel bus and the same clock. The best example I can think of for use in Minecraft would be buffering the input to a binary decoder/encoder circuit, such as a seven segment display circuit. This use would be of dubious value as well though, as you would typically have a T Flip-Flop counter feeding the circuit, and it's just as easy to add dumb delay (i.e. repeaters) in order to synchronize the counter outputs. Again, a D Latch (which uses just two repeaters) is more appropriate here than a full flip-flop.
Going back to the shift register example, a full D Flip-Flop implementation usually isn't necessary; using inverted one tick pulses to a D Latch will make it act as a D Flip-Flop. Knowing this, it should become obvious that a full D Flip-Flop implementation usually isn't required, and a D Latch (plus appropriate pulse limiter circuitry) can be used instead.
From my own personal experience, I tend to use T Flip-Flops (either individually or in a counter chain) and RS-NOR Latches in any circuit I build, and those are the components that I would recommend learning after you know how to build the basic logic gates. From there, I would recommend learning pulse limiters, pulse extenders, and edge detectors. That doesn't mean there isn't a place for D Flip-Flops, but their utility is often limited to very complex circuitry where input synchronization is absolutely necessary, or where parallel to serial conversion, or vice-versa, is required. That simply doesn't come up that often in Minecraft circuits.
