My first career was mechanical engineering and this was my go-to source for any prototyping supplies. Their catalog is extensive, but more than that the data the have on every part (and allow you to query) is incredibly detailed and useful - basically exactly what a designer would need.

Say you want a bolt - you'll need a certain material for the application because maybe it needs to be corrosion resistant, or have high strength - you can filter by that. Then whereas other places will let you select by thread size, McMaster has selection for overall length, thread length, head type, tensile strength, self-locking, you name it. All these details that make it incredible useful to incorporate into a design. And then each part has a fully dimensioned drawing with tolerances and callouts so there's almost no ambiguity! Who does that?

McMaster-Carr is really a master class in UI when it comes to shopping for technical products/materials. It's clear they put in the effort.