I Want It Cheaper!
When you buy from an online-only business, they don't have the same expenses as your local brick and mortar (B&M) shop down the street. Rather than having sales people on hand to answer your questions and store space in an area that's convenient for you to get to, all they need is a warehouse and a website. Additionally, taking orders online often allows them to handle a greater volume of sales, which means they can get by on a lower profit margin than a traditional store would need to stay in business. This is one of the main advantages of online shopping: it can be a lot cheaper!
However, that doesn't mean you should get carried away. For many things, the added aggravation may not be worth the cost savings, and some things are best done in person. If you're buying a new suit, for example, you want to be measured by a professional so that it can be correctly fitted, and if you don't know exactly what you want, a trained salesperson can help you figure it out.
Buying online is best suited for commodities: things that are the same no matter where you get them. If you're looking for a book, for example, and you know which one you want, Amazon is probably the cheapest and easier way to get it. On the other hand, if you're looking for a painting to hang in your house, you'll probably want to visit local art galleries to find something you like.