JMO:
In my area, pools are frequently built near steep slopes, or in a grade, or near existing structures (e.g. houses or retaining walls), or in areas with poor drainage, or in poor soil. Site drainage can be dramatically affected by the project, with negative con$equence$ for neighbors.
There are many instances where engineering is important. For example, it might be important to evaluate soil type, soil stability, and soil moisture content for the pool site. Engineering might be needed to characterize the site and determine the need for piers or chemical injections or drains. Structural engineering might be needed to assess loads on pool walls (and bond beams) or loads on new/existing retaining walls and to design/specify structures capable of withstanding these loads. It is quite common for engineering analysis/design to call for changes to steel/rebar, beam/wall/cove/floor gunite thickness, pool plumbing, and site drainage.
PB's usually quote outdoor-scape design (form/appearance/fit/function) and construction, but not engineering. In one way or another, the PB's contract will include clauses which (1) place the responsibility for engineering or for opting to not engineer on the buyer and (2) place the liability for consequences of construction on the buyer. So it is important for the buyer to be able to identify situations in which the need for engineering is indicated.
IMO, engineering fees are usually quite reasonable relative to the overall cost of the project and well worth the cost.