I am new to instrument flying and am trying to wrap my head around the different types of GNSS approach minimums. I am having an issue understanding why LNAV minimums exist. My understanding is, if you're GPS is not WAAS-certified, then you are flying your approach down to LNAV/VNAV minimums. In this case, the lateral guidance is provided by SBAS (your GPS), and the vertical guidance is provided by your barometric altimeter. So...my confusion here is...since most aircraft have barometric altimeters, when would you ever use the LNAV minimums? Since every aircraft contains some sort of altimeter (your vertical guidance), then why does an LNAV approach even exist? Is it only for the case of an aircraft having a GPS and a non-barometric altimeter (i.e. a radio altimeter)?
In LNAV/VNAV approaches, the vertical guidance can be provided by WAAS, or it can be provided by a baro-VNAV system.
On an LNAV approach, there are step-down waypoints just like on any other non-precision approach. The pilot uses the altimeter to manually adhere to minimum altitudes.
On an LNAV/VNAV approach, the avionics compute a glideslope and provide vertical guidance all the way to the decision altitude, so the pilot can "chase the needle," just like a precision approach. The vertical guidance can be provided either by WAAS or by the barometric altimeter ("baro-VNAV"). Not every airplane with a barometric altimeter is capable of using it in this way.
So LNAV approaches are the only ones that can be done by aircraft without baro-VNAV or WAAS available. There also can be restrictions on the other approaches- for example, baro-VNAV approaches are often not allowed if it is too cold, due to the altimeter errors caused by low temperatures.