Buying a bike, I almost always buy locally. I return a lot of stuff I buy online via amazon etc when it shows up but doesn't quite work. A bike is way to much hassle to send back. In person I take it for a test ride and get an idea of:
- If it's the right size
- How the ride feels - I've test ridden a few "knock your teeth out with chatter" bikes and one "expensive super smooth riding bike" that had a very mediocre ride

I don't really care about the cost difference. I feel like the really big cost differences a lot of times comes with a crappier bike. Like yeah you can buy a much cheaper full carbon bike online, but how does the cheapest model ride? You can make a carbon fiber frame that rides really awful.

For accessories and parts...a lot of times I buy them online despite that I'd theoretically like to buy them in person. Some things like bike locks the price difference is huge. A lot of other times they just don't have what I need. For example I wanted to buy a "Planet Bike Grateful USB Bike Tail Light" because I read they added a reflector for it for times when it's not on. My closest bike shop doesn't sell it at all. The next one you can order it online, but only if they ship it to the store, so I'd have to order it, wait for it to arrive, drive to the store during business hours...I gave up and went and order it on amazon where it will be on my porch in 3 days.