There used to be two advantages to resin. First, the investment to make molds for resin casting is minimal compared to the molds and other aparatus for injection (styrene) molding. Thus, there were many cottage industry firms that made small runs of obscure subjects, if you want something different than another ME-109 or P-51 varient you could chose from the many resin offerings.
Second, it used to be the low surface tension and viscosity of urethane resins, and the detail obtainable in RTV molds allowed detail with resin casting unobtainable at the time in injection molded styrene, if the kit developer put enough effort into the pattern (original from which mold was made). However, in the last few years new technologies have allowed larger styrene kit mfgs to produce equal detail.
In summary, well made resin kits can have detail as good as or better than injection molded styrene, and the large number of small garage-shop mfgs that can get set up in resin means a wide variety of subjects. Not all resin mfgs, however, have, or can afford, skills to make high quality patterns/originals.