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.