For some tasks, surveys in particular, field of view is critical. You would need many Hubble telescopes to do what Roman can do mapping everything out there.

"What really makes Roman stand out is that it is designed for wide-field imaging. The cameras on telescopes can often only detect objects in a very small patch of sky. If we use the size that the full moon appears in the sky as our guide, Hubble’s field of view is often smaller than that —meaning each picture it takes is of a patch of sky smaller than the full moon. Webb’s instruments often are even smaller than Hubble’s. Each picture taken by Roman, however, will be larger than the full moon, allowing it to survey more of the sky much faster than Hubble or Webb."

https://science.psu.edu/news/ask-expert-will-roman-help-reve...