High precision resistors are often necessary for metrology applications like very precise and low drift voltage sources. Often parts like Vishay's same-substrate thin film resistor networks [0] are used, as the temperature of each resistor leg are kept the closely relative to each other, resulting in the ratio between them being stable against temperature changes. Even if you use some adjustable/tunable circuit, you usually still require some sort of precision resistor network as an original standard.

In general, however, it's much better to measure/sense physical phenomenon by first converting it into frequency, because it is much easier to measure frequency precisely. Using something like a TCXO from Seiko Epson with 1 ppm tolerance, and measuring over time, you can easily achieve 0.00001% precision and beyond. I know that strain gauges used in civil engineering often utilize this concept, where a metal string is "plucked" electronically and the frequency is then measured.

[0] https://www.vishay.com/docs/61010/ccc.pdf and https://foilresistors.com/docs/63120/hzseries.pdf