I assumed everyone knew this, but I had two different people tell me otherwise in game last night until I forced them to go test for themselves. So, just posting in case it's news to someone.
Guardian's Favor *does* have a purpose in DoL Leves.
For a DoL leve that has you gather 3 rare items, Guardian's Favor will increase the SP gain for successfully harvesting one of the rares by 50%. I.e...if a L20 or so Miner is doing the Rich Iron Ore leve you might get 400SP for each rich iron ore without favor, but 600XP per with favor.
For the DoL leve that has you deplete 3 promising locations, you get 50% sp bonus per successful harvest at each of the 3 locations. Again, with R20 or so miner, you might be 120SP or so per successful rank 3 dig. If doing the Leve, you will get 180SP or so per successful dig as long as you have favor on.
Basically, the "3 rares" leve is stupid good xp per minute and should always have favor. The "promising spots" leve is good to have favor on if you can reliably harvest successfully.
Favor does not increase the SP you gain from a failed harvest.