what's the purpose of parallel RL circuit on mains recifier?
I believe these two filters were intended to smooth the fast rise times present on the AC source voltage, probably to prevent disturbing the SMPS on each half cycle.  Thus the two, 2-pole filters from AC in to DC out.

The LT3799 is a flyback SMPS controller, but it's intended specifically to drive LEDs.  One of its features is the ability to be powered from a phase controlled voltage source (triac dimmer).  The DC1744A board was designed to demonstrate this among other features.  Triggering the triac at 90 and 270 degrees (worst case) subjects the input to a step of around 165V.  It's this step that I believe the filters are trying to tame (my opinion...I didn't design the demo board!).

The two 10k and the 750 ohm resistors I believe are as Tony Stewart pointed out: they lower the Q so that the input step doesn't cause severe ringing.

Another feature of this IC is active power factor correction.  For this reason, I don't believe the R-L-C components under discussion are intended to correct power factor.