Because they need more specific information than the NWS provides, or they want meteorologists who may in fact be better at their craft than the NWS ones (not a slam on them as a whole). Read on...
In the New Orleans area, there was a phenomenal meteorologist, Nash Roberts. He started out doing weather forecasts after WW II for oil companies doing drilling in the Gulf of Mexico. They have billions of dollars of rigs and equipment and thousands of people working the rigs. The related support companies who serve the rigs have thousands more. They need someone who really understands the weather from their perspective.
In the early 1950s Nash went to work for WDSU-TV in New Orleans, though later was hired by different stations.
in 2004, he was the only one to accurately forecast what track Hurricane Katrina was going to follow. The other weather locals at first almost made fun of him but had to eat their words despite their computer superiority (this was not the first time).
He actually made it a point to understand the weather and not just read NWS copy though he thoroughly respected it. Locally, "What does Nash say?" was a real question based on his decades of accuracy. His forecasts were not always perfect, but accurate; he learned from erroneous forecasts and shared that. He was genuinely honest and a nice guy.
Re Katrina, a common question in New Orleans about any possible hurricane approach was "Should I leave?". Nash's honest answer was if he told his wife to leave, then get out because he was always going to protect her. He told her to leave for Katrina, and shared that with the public.
This is what I know from having been born and raised there for 25 years, though now living in the Atlanta area for 40+ years. And am still a weather junky of sorts ;o) in part, thanks to Nash Roberts.
Others here will be able to address other meteorological specialties, such as flying where you need very detailed reporting higher in the atmosphere. The NWS probably has that info, but a reliable specialist may interpret the data differently and potentially more accurately.
HTH,