The Simple Text block allows you to use a Web Engine block with a smaller footprint on the webpage. The RTE is more MS Word like, but the simple text block uses less space in the overall download, so we should try to use this block where possible.
Also, the simple text block has some formatting options for the text that the RTE does not. So consider this when there are smaller sections of text that do not require a word processor like functionality of the Rich Text Editor.