The overall quality of Arduino connected hardware designs varies widely as the community consists of a comparatively large number of hobbyists and students.
The 0.1uF cap was most likely intended to decouple the adjacent +5V power supply line. Attaching it to the latch signal line serves no unique beneficial purpose and can cause problems if your firmware operates the clock line at its maximum frequency and you have a long serial chain of registers.
If you need to delay the latch pulse to comply with setup and hold times (timing minimums) of the 74HC595 you are using, then you should address this in other ways: either correct your layout and/or add the delay in your firmware.
On its necessity...
jippie said it best:
The type of bouncing where this configuration works, is not a digital controller output like the your diagram implies. It is more intended for a push button.
On its consequences...
Even if we believe that a bouncing source was driving this line, there is no practical consequence of such behavior.
Wouter van Ooijen, said it best:
Debouncing the (data) clock input might serve some purpose. Debouncing the latch (clock) input is nonsense: multiple pulses on this input will simply re-load the same data from the shift register to the holding register (latch). -- Wouter van Ooijen
Some schematic programs use this as a symbol to show that the pin is specifically not connected to anything.
However, this looks to be a schematic from Eagle. Eagle requires no such symbol. Output or passive pins are allowed to not be connected without any electrical rules check error. You can make a symbol and footprint for anything you want in Eagle, so only the designer knows for sure what this means. It could mean a test point, but most likely not since there is no component designator. A bunch of test points on the board are pretty useless unless labeled, so I don't think that's it. Most likely whoever created that schematic was used to a different schematic program and felt compelled to put Xs there to indicate not connected. In Eagle, you simply don't connect anything and don't give it a name and it won't be connected anywhere.