I think the other replies to you are good, i'll just add,
unlike BMI, waist to hip focuses on abdominal fat and the additional claim is that abdominal fat is "the worst kind" for health, i.e. (i'm not an expert, and i'm not going to research, but from memory) men tend to get belly fat, women tend to get hip fat, but women who get belly fat are at increased health risk in the same way that men are, heart attacks, etc.
I linked to one review earlier, but there's actually a lot of evidence that WHtR is quite good against a number of populations and ages, and certainly for each individual, even if you ignore cutoffs, it will be directionally correct:
Corbatón Anchuelo, Arturo, María Teresa Martínez-Larrad, Irene Serrano-García, Cristina Fernández Pérez, and Manuel Serrano-Ríos. “Body Fat Anthropometric Indexes: Which of Those Identify Better High Cardiovascular Risk Subjects? A Comparative Study in Spanish Population.” PLoS ONE 14, no. 5 (May 23, 2019). https://doi.org/10.1371/journal.pone.0216877.
Djap, H. S., B. Sutrisna, P. Soewondo, R. Djuwita, K. H. Timotius, Trihono, S. Sharif, and Y. S. Tjang. “Waist to Height Ratio (0.5) as a Predictor for Prediabetes and Type 2 Diabetes in Indonesia.” IOP Conference Series: Materials Science and Engineering 434 (December 2018): 012311. https://doi.org/10.1088/1757-899X/434/1/012311.
Zhu, Qihan, Feixia Shen, Tingting Ye, Qi Zhou, Huihui Deng, and Xuejiang Gu. “Waist-to-Height Ratio Is an Appropriate Index for Identifying Cardiometabolic Risk in Chinese Individuals with Normal Body Mass Index and Waist Circumference.” Journal of Diabetes 6, no. 6 (November 2014): 527–34. https://doi.org/10.1111/1753-0407.12157.
Vrdoljak, Davorka, Biserka Bergman Marković, Ksenija Kranjčević, Dragica Ivezić Lalić, Jasna Vučak, and Milica Katić. “How Well Do Anthropometric Indices Correlate with Cardiovascular Risk Factors? A Cross-Sectional Study in Croatia.” Medical Science Monitor : International Medical Journal of Experimental and Clinical Research 18, no. 2 (February 1, 2012): PH6–11. https://doi.org/10.12659/MSM.882451.
Ochoa Sangrador, C., and J. Ochoa-Brezmes. “Waist-to-Height Ratio as a Risk Marker for Metabolic Syndrome in Childhood. A Meta-Analysis.” Pediatric Obesity 13, no. 7 (2018): 421–32. https://doi.org/10.1111/ijpo.12285.
Caminiti, Carolina, Marisa Armeno, and Carmen S. Mazza. “Waist-to-Height Ratio as a Marker of Low-Grade Inflammation in Obese Children and Adolescents.” Journal of Pediatric Endocrinology & Metabolism: JPEM 29, no. 5 (May 1, 2016): 543–51. https://doi.org/10.1515/jpem-2014-0526.
unlike BMI, waist to hip focuses on abdominal fat and the additional claim is that abdominal fat is "the worst kind" for health, i.e. (i'm not an expert, and i'm not going to research, but from memory) men tend to get belly fat, women tend to get hip fat, but women who get belly fat are at increased health risk in the same way that men are, heart attacks, etc.