an online store

2 basic reasons:

Attention span.
Page load times.

The public has a limited attention span. They want to find what they are after with a minimum of fuss, delay, and hassle. It needs to be 3 clicks or less to find what they are looking for. Ifthey haven't found it by the 5th click, they will go elsewhere.

Since most potential customers are using dial-up connections, a simple web page is required to keep load times to a reasonable length.
If the page will load in under 15 seconds at 28.8, the customer will stick around.
If the page takes 3 minutes to load at 56K, the customer will become bored, and go elswhere.

So, it boils down to keeping the customer active and happy. Just like any other retail situation.

Chas