For me, it's all about convenience. I absolute hate cloth shopping. Now I can order a bunch of stuff, even the same thing in different sizes, try them at home and ship everything I don't want back.
And also with electronics, why should I go to the store if I receive the package the next day?
There's another convenience factor here: being in-stock.
With B&M shopping, even if you know exactly what you want, you have no idea if it's actually in-stock at your local $store. So you drive there (wasting gas and a lot of time), wander around inside looking for it, only to find it's not even there! Then you have to go drive somewhere else hoping they have it, or get someone there to look it up and see what other store does have it, etc. All in all, this can waste a huge amount of time and fuel just to go find one single item.
With online shopping, it's right there on the website: you can see everything the shop has to offer in one place, and you can see if it's in stock or not. If it is, you click "buy" and it shows up at your doorstep. If it isn't, or there's some problem, that might mean a delay in getting your item, but you're not going to waste a lot of time and energy running around in the process.
And many stores' customer-facing inventory systems are not reliable, or only updated every 24 hours. If there is only 1-2 of an item in stock, there's a decent chance it's actually sold out.
So if I'm already looking at the webpage, why bother driving to the store to get it when I can just click "buy" and be done with it? The only reason I'd bother with driving there in person is if I really really really need it fast.
And as the other responder said, those inventory counts might not be accurate; if there's only one in stock, and someone just bought it 10 minutes ago, is the webpage really updated that fast?
Because I get same day shipping at no extra charge. I've done it myself a few times. It is really nice to walk in and have everything waiting for me (this is even more important now that my kids are of the age to fight over who gets to push the cart until they dump it). Better yet, if they don't have it at that store I can go to the next store, their competitor, or pick a different model they do have in stock.
Amazon uses fast shipping, but they won't guarantee when it ships. I've been burned more than once (I mostly quit using them a couple years ago) because the thing I wanted didn't ship in time.
Ebay offers fast and free shipping on many items with guaranteed dates, usually 2 or 3 days, without having to pay for a prime membership.
I've seen online shipping speeds get a lot faster over the past couple of years so if you have quit than you might be surprised at speed and value if you come back.
Hmm, seems like Europe is further ahead in this field (I'm in Prague, Czechia). I can order the item and have it in less than 6 hours delivered to my doorstep and professionally installed, guaranteed. It costs just around triple of what would driving my own car cost.
That is available in some parts of the US as well (I have friend who get 2 hour delivery). I live "in the middle of nowhere", I can get to a big store in half an hour, but we the town is no big enough that anybody has got the fast delivery model down yet.
And also with electronics, why should I go to the store if I receive the package the next day?