Looks like a syphon break. I may be wrong though, I'm still new to the plumbing aspect myself.
If the water line lowers below that point, in the case of a power outage, then your siphon will break and stop water from flowing to your sump, preventing an overflow/flooding.
Definitely good.
I'll let someone else chime in about the skimmer. I'm no expert, but I know if there's a piece of equipment to invest well in, it's definitely the skimmer.
The seaclone skimmers are one of if not the lowest end skimmers on the market. Mine crapped out on me after two months and never really pulled anything. I've switched to an eshopps and it works 1000 times better.
Check out some reviews online. I read a few and some were actually surprised compared to the negative reviews it got. You can always play it safe and return it, if possible, and dish out a little more and pick up a more notable skimmer.
Have you looked at the reef octopus skimmers?
Jesus...that is double the size of mine! 400 gallons. You're moving in the opposite direction of size. Mine would work well on your system, but to fit your budget listed just take a step down.