Garo Yami wo Terasu Mono 19 released

[LightningZERO]

I'm more inclined to believe the writer didn't pay attention to the first two seasons >.>

Actually no, the writer did his/her homework.

While it is not stated in the first or second season (as far as I can tell), paying one day of life to Zaruba in exchange for his service is actually part of the lore and can be found in supplement Garo guidebooks. I remember reading about this in chinese forums few years back and did some quick searches. You can actually found japanese forums/blogs talking about this setting back in year 2011.

[Lunagel]

Okay, I'll trust your information then, but it still doesn't explain what exactly Garo gets out of the deal.

[takenoko]

Besides, the main writers of Yami are the same as before. Taguchi Kei helped co-write the original Garo series with Amemiya Keita (along with other writers). Era Itaru has been the main writer for Garo from Beast of the White Night on. These guys would know their Garo lore.

[LightningZERO]

Okay, I'll trust your information then, but it still doesn't explain what exactly Garo gets out of the deal.

This I agree...seems like a steep price to pay for a navigator. But then again Zaruba does have knowledge on Makai stuffs that might be useful in the field, and if you are close with him enough, maybe he can help you by pulling that nice trick he did in the last episode of season one.