Why did he leave her behind? I thought he said he'll just check things out

That is probably one of those skipped double explanations when novel put parts of information out just to later repeat it full. Without those it really sounded like you say.

Whole time, ever since he trained that skill, QC had strange feeling he is missing the clue. First was the skill they trained outside, then it was sword intent, then spear and each time he was bugged into "wtf is wrong here?". First reason why he left her was to go and check wtf is going on up to the top. Second reason was that he realised that girl is granddaughter of Ji Wuxue.

Also, these episodes now have some importance.

What happens on top is the part of arc2 introduction (ancient war). So far, there were few clues into it. Jade tower, competition in ancient ruins, illusion ghost in the swamp and everything with skeleton helmsman. By the time arc2 starts, you already get to know ins and outs and who is who, so arc2 just starts full speed.

Second important thing is that with this episode, there was start of Ji family sidestory which spans all the way until end of arc3

Next

Probably another waifu has join the list.

Next

Imagine that was how you learned