There are a million reasons for online lessons, but perhaps this resource reminds why we access resources online and use them for teaching. What others can create to share with us to better or further or understanding of complex topics is amazing. Here is a great example of a Biology concept, to help students better understand the cell size and scale.
Utah State University put together a great simple Flash object, a student can zoom in and out of different items to compare there sizes. If that was not enough, one can look in the upper left hand corner to see these items on a 1 mm graph paper scale. Not only are we learning about science, but math as well.