Why might a placoderm be more useful index fossil than a cockroach?

Placoderms are an extinct class of prehistoric fish, whereas cockroaches (order Blattodea) very much exist and thrive today. The placoderms are only known from fossils, and were armoured prehistoric fish which lived from the Silurian Period to the Devonian Period. So we only know about them by their fossils, making their fossils very important. The Placoderms were among the first fish to develop a jaw and pelvic fins - the precursor to hind limbs in tetrapods, and this in itself provides very important information regarding the evolution of fishes and tetrapods, including ourselves.