McMaster's marketing is done well before you type the address into the browser.
Their target custom is someone who needs to quickly get the item so they can go back to their job; make and price are not their concern -- to the extent that you can just email them and say "give me five more shelves" and their customer support people will look through your purchase history to match the model and paint scheme you bought. Or if you're a power plant, you can tell them to get the part to you RIGHT NOW, and they'll pay a courier to drive it from hundreds of miles away. (These are both true stories.)
McMaster's customer behavior is the dream of almost every industry out there.
The added value of this site for a non-native speaker of English is that we get to learn the proper words for the technical things you never know how to call.
It starts with a picture and then you get to the gory details.
Sometimes I want to change industries just so I can order from them all the time.
(Some other business-getting-work-done sites are similar, https://www.uline.com or https://www.grainger.com but they have color and can't compete with McMaster Carr.)