I can't count the number of times when I bought a motherboard, and didn't realize my old AT power supply no longer worked with an ATX motherboard, or some other random standard, connector, or form factor changed. You can ask an employee "Will these five parts make a computer?" and get an answer, or even configure one up on their web site.
Plus, sometimes there are defective parts. You buy a part that doesn't work. You plug together motherboard, RAM, CPU, and power supply, hit the power switch, and very little happens, with no real way to debug. I won't say that's easy or seamless with Microcenter, but if you have five parts from Newegg and Amazon, you're basically SOL.
And if you forgot something -- and I've never built a computer where I didn't forget something -- you can pick it up. All the little things are just easier. Building a PC requires a ton of screws, cables, and adapters. I've never had the right set come with parts. Sometimes, the motherboard, case, and peripheral will all come with the same $0.50 part (Yay! I have two extra I don't need), or none will.
With COVID19, I've missed Microcenter; they don't do curbside pickup, and generally have avoided taking any sorts of basic safety measures. It's odd.
It's a fantastic store, but it very clearly hasn't had a remodel since it opened.
Unfortunately my Frys has been crumbling for years (nothing says “commitment to excellence” like dozens of buckets around the store capturing rain water), and I finally gave up going.