There are no apparent such things as "Atlanteans" other than those who live in Atlanta, GA. As to the "why" question, it is apparent that the pyramids hold a funerary purpose and that the elite rulers (pharohs) believed it their right to be interned in monumental fashion for their transitions to the afterlife. The pyramid of Khufu has an observable transition from early mastabas that began with single story, mud-brick structures and gradually increased in size and grandeur through Imhotep's design of Djoser's (Zoser's) "Step-Pyramid" at Saqqara to the "Bent Pyramid" to the major structures of Giza that come to mind for most people. There is no evidence that the 51 degree angle of some pyramids is anything but a convention of simplicity and common sense. Pyramid angles vary and, indeed, the "Bent Pyramid" has multiple angles (51 and 43). Very likely the pyramid angles were meant to provide maximum height/grandeur and minimal chance of collapse. Also, the steeper the angle, the more difficult it would be to transport blocks up ramps. The brightness of the pyramid itself added to the grandeur and monumental appearance, however. And they would have been visible for miles, particularly with their white limestone casing stones in place. These stones, however, were nearly completely removed in later periods to construct fortifications. According to Petrie, there were no writings or inscriptions on any of the casing stones of the Great Pyramid or those at other sites (i.e. Medum, Dahsuhr, etc.). It would appear that your "source" is flawed. Moreover, the esoteric "law of the spirit" notwithstanding, the ancient meanings of the pyramids are clear: they're funerary. The "why" involves "why" tens of thousands of citizens were motivated to support the authority and validity of the elites? One answer might include the association of the elites (the pharohs) with gods and the divine, but it would seem that these "gods" must have been able to provide some assurances that their divinity was genuine. Their apparent control of the Nile? Prosperity and success of harvests and military?