Many foundation repair companies take elevations (called differential elevations) when they prepare an estimate to perform foundation repairs. This can be an effective sales tool. Some companies represent that there are maximum slopes (or deflection values) set by VA, FHA & HUD for existing buildings. If I recall correctly, it was 1947 when the FHA produced minimum standards for new home construction. I have yet to see any official document produced by any of the agencies on slopes or flatness for existing house floors.
       Most elevations taken are not documented as to the exact location, so they cannot be verified by a third party. To be verifiable they must be documented by showing exactly where they were taken (within a few inches). A floor plan can be used to show where they were taken. This entails measuring the existing walls and creating the floor plan for the sketch, documenting the locations and measuring the elevations. It also has to take into account the floor covering thicknesses to be able to establish the precise differences to a common baseline, such as finished surface of the constructed slab. These details require time, and therefore, additional costs.
       Some engineers take elevations to get a feel for the general condition of the foundation. This assumes that the original construction was flat and level. That is not always the case. When an engineer discusses deflection values of L/360 or L/240, he is using design criteria. This originally was established for the purpose of uniformity of minimum values for deflection in beam design. That is not an official criterion for existing foundation slopes or deflections.
A highly trained engineer or technician can evaluate an existing building without taking elevations. Some individuals have been gifted by God with the ability to feel the slopes of floors with such precision that they are able to estimate the drop across a room within 1/2".  Most who do not have that gift won't believe it exists if they don't see it demonstrated, and scoffers will still deny it.

In my opinion there are three reasons that a person may want differential elevations in a report:
1. If there were elevations taken in the past, and there is a need to determine any changes; this requires that the locations are sufficiently documented so it can be verified by a third party.
2. If there is a need to establish a base line for future differential elevations.
3. If I have to appear in court to testify that there are indications of movement, I have to be able to show exactly how I reached those conclusions with a methodology that can be verified by a third party.