Why requiring input resistors in series before digital-to-analog convertor (DAC)?
More than likely there to reduce EMI.

I had to do that extensively on video cards I designed to slow signal rise and fall times to limit the harmonics and reduce the EMI that would couple into, and be very visible on, the video image. 

Generally speaking, whatever is driving a digital line has a much faster edge than the thing receiving it needs. This is because the driver is designed to be capable of driving many gates. As such, you can have quite a bit of room to slow down the edges. This is especially true of synchronous bus transactions that have well defined setup and hold delays.

The resistor, along with the capacitance of the trace beyond it, act as a low pass filter.

It makes quite the difference.