Number one reason Lowe's and HD are my usual stops for electrical is because they are, by far, the closest to the jobsite for things not on the truck already, and I just do too much different stuff to have a fully stocked truck, especially since most of what I do is air conditioning, and that's what I stock the truck with and those have to come from a real supply house anyway. Lowe's gets the nod over HD most days because even though the latter has a bigger selection, Lowe's usually has what I can use, and I get an everyday 10% veteran's discount.
I can be in and out of a big store much more quickly than I can go to the supply house and stand in line to TELL them what I want. Half the time, I'm looking at the parts on the shelf in order to decide HOW to do a job with what's available.
The big stores certainly have their faults, but the advantages are also significant. Price differences are often the least of my concerns.
Often, I can schedule a job for a couple days away. In this case, Amazon Prime is my usual source, and I skip the store trip entirely.