Key advantages are ability to customize, far better warranties and reduce "buy cheap, buy twice" false compromises.
Some observations:-
- OEM's pay Microsoft far less for each bulk Windows license than the end user retail price of $90 per PC. That's what skews much of the "OEM vs self build same component" price comparisons. If you already had a legal retail (non-OEM) Windows 10 license that could be transferred across hardware upgrades, your $870 self-build price would fall by $90 to $780 whilst you may only see a $20-$30 reduction for the OEM if a "bare" OS-less OEM version were to be made available.
- Cheap & nasty fans. As usual for OEM's, they stick high RPM 3-pin fans in with poor fan control. Example reviews from the PC you selected :
"Hello I purchased this computer. But the fans are super loud" / "The way this PC is built is the result of the noise. The fan thats so loud is a case fan. It is a 3 pin connected directly to the power supply, meaning it runs full speed always. Now if it was a 4 pin you could hook it to the motherboard, and then control the speed through the BIOS and things like that." OEM's are literally plagued with dumb design decisions like this that "over-err" on the side of caution.
- Cheap & nasty CPU cooler :
"Does this come with the stock Wraith Spire CPU cooler that normally comes with the Ryzen 5 1400 CPU?" / "No it doesn't. I was a little disappointed by this. It just comes with a cheap tiny heatsink and fan. No brand name on it." / "On a side note: I'm extremely disappointed with this computer, because of the building flaws, the cheap CPU fan (non Ryzen), the outdated Bios" / "the Bios will give you a CPU temperature that is inaccurate"
- Poor warranty. Basically 1yr (all components inc PSU) which is voided for the whole rig if you overclock. Compare that to retail components where you'll often get 3yr for MSI motherboard, lifetime RAM, etc.
- 1x single 8GB chip means no dual-channel, plus increasing low RAM problems on the latest (and future) increasingly bloated games. Buying another 8GB RAM chip in 6-18 months time then "forgetting" to price it in is not being honest as to the real TCO vs a self-build that starts out with 16GB RAM.
- Cheap Bronze rated Corsair "CX" PSU's are just not that good. You already know this as you (wisely) plan to replace that too, but again "forget" to price in what you've already spent. Decent 7-10 yr warranty Gold rated PSU's with 105c rated caps are far more likely to last 10 years whereas this CX PSU and cheapo 85c caps may only last 3-5 years (hence the massive disparity in 3yr vs 10yr warranties).
This whole PC (inc PSU) has just a 1 year warranty and since it's sold as a pre-built, quite often OEM PSU's have visibly different designs or labels on that cause manufactures to reject them if returned faulty as a retail after the 1yr OEM warranty is up. How much did you pay for your "good 650w PSU" and how long is the warranty? Or alternatively, price in a second spare CX to compensate. It's all this stuff that highlights the false economy of 1:1 comparisons of cheap OEM components vs much better retail components with much higher reliability and 10x longer warranties.
- A $36 Wi-Fi "N" adaptor which doesn't even have AC isn't a particularly good buy in 2017. When I bought an Asus B150I motherboard, there was less than £10 premium between the Wi-Fi and non Wi-Fi versions, and that was for 2x2 AC (866Mbps) that's double the speed of 3x3 "N" (450Mbps).
- You also may not need exactly the same model of components if another cheaper one will suffice. Eg, you picked a motherboard with 4x RAM slots then stuck 1 RAM chip in it. Another board that's cheaper with only 2x slots may still be perfectly acceptable. Another $10-$15 saved off the self-build version.
- $400 for 4GB VRAM mid-range GPU's is just a silly buy at the moment. There was at least one "In Stock" EVGA GTX 1060 6GB listed on ncixus earlier for $290, so that's $110 saved off the self-build. If you can't find sanely priced cards when they pop-up (typically faster than PCPartPicker tracks them), then you'd be far better off spending money on stuff that isn't wildly over-inflated due to the current "mining tax inventory starvation" thing, like 16GB RAM, a better CPU, bigger SSD, etc, now and then buying a GPU later on in the year once inventory issues have eased up and prices return to normality.
So far just reading the user reviews you'll need to at least factor in and replace new PSU, case fans and buy a decent CPU cooler (on top of replacing the HDD with an SSD and maybe the "N only" Wi-FI with AC), and hope that the motherboard uses the standard BIOS and not an OEM-specific one before finally accepting a warranty that's 3-10x shorter than what buying same components retail provides. And don't overclock if you want to keep it. Suddenly the $120 savings (or just $30 if you already own W10 retail) don't sound particularly "cheap"...