Was there an issue with targeting sub-populations for clinical trials beforehand?
At an ELI5 level, if you're hoping your drug candidate will help cure disease X, you sign up patients with disease X to join your clinical trial. That's not the hard part!
(source: family member works at $bigPharma)
a) If you suspect there is a significant pharmacogenetics component to what you are studying (or related to disease progression).
b) You're working on something preventative.
From a previous job I worked at (we made PCR tests), there was interest on screening for APOE genotypes to enrich an Alzheimer's drug trial - drug maker believed that APOE genotype would have a significant impact on drug performance.
But you're absolutely right that you can often (usually?) do 'enough' enriching without genetic information.
An example of this is when they tested the covid vaccine, they wanted to make sure they had enough participation from African American and other ethnicities since these were usually under represented in clinical trials relative to the population, and there are sometimes subtle variations in the way peoples body respond to drugs depending on their race.