Think about what makes VDRL and RPR clinically useful. These reaginic tests measure antibody to a cardiolipin antigen, and crucially they are reported as titres obtained through serial dilution. Being quantitative is their defining strength, and from that single property flows their main clinical use: serial titres can be compared over time, so a four-fold fall confirms cure and a four-fold rise signals relapse or re-infection. Hence both quantitation and treatment monitoring are genuine benefits.
The other two choices are distractors. A single standard dilution is not how these tests work - they are titrated across multiple dilutions. Lifelong positivity is the hallmark of treponemal-specific tests (TPHA, FTA-ABS), whereas non-treponemal titres wane and may sero-revert after adequate therapy, which is exactly why they are useful for follow-up rather than a drawback.
\[\boxed{\text{Quantitative} \Rightarrow \text{monitoring response to treatment}}\]