Because Amazon is likely to have what you are looking for at a substantially lower price and if you need to return something (pretty much anything for ANY reason) they make it extremely simple?

Depending on what it is I'll try to buy from a local business first but if it is 30% cheaper on amazon or it is something that might need to be returned I'll be looking at Amazon.

Case in point - I ordered a 64GB memory card for my g/f's daughter for Christmas. I used the address for her that was stored on Amazon from previous purchases. Amazon/UPS reported it delivered last week. Christmas morning I find out that she never received it. Thinking it was probably stolen from in front of her door where UPS said they delivered it I got in touch with someone on live chat at amazon. Christmas Day I had a live person within 20 seconds. After letting him know she didn't get it (probably stolen from in front of her door) he said he would overnight a new one free of charge. He then asked me to verify the address. I asked my g/f to verify it. She said "That is her OLD address!". I sheepishly let the Amazon rep know that oops, "I" made the mistake and it was my fault. He said no problem, he would still send a new one free of charge overnight to the correct address. I don't know of ANY other company that would have done this. She received the new memory card this morning at 9:30 am.

For something like my bike I chose a LBS over say bikes direct for the face to face interaction. Some items don't need that.

Electronics? Amazon will beat Best Buy any day.