This command will be useful in many ways. The Pixelmon team themselves can use this command to test out bugs and bug fixes related to raids. Players and server owners can probably use this command to test out any modifications to the raid .json files or changes to the raid config settings.
There are a few different potential arguments to the /raidbomb command that I have thought of:
/raidbomb
Using the /raidbomb command with no arguments will spawn a Pokemon in every Max Raid Den in loaded chunks, and it draws from the same pools of Pokemon that would be drawn from if a player throws a Wishing Piece into each of those dens. The chances of the Pokemon being Shiny, Gigantamax, having their Hidden Ability, and having a particular Star Level are the same as they would be if the player was throwing a Wishing Piece into each of those dens.
/raidbomb 5
Using the /raidbomb command with an integer from 1 to 5 as an argument would spawn a Pokemon in every Max Raid Den with the same Star Level as the number used. In the above example, every Max Raid Den will spawn a Pokemon with a Star Level of 5, from the pool of Pokemon with Star Level 5 in the biome where the Max Raid Den is located.
/raidbomb pixelmon:ultra_deep_sea
Using the /raidbomb command with a biome as an argument would spawn a Pokemon in every Max Raid Den drawn from the pools for the biome that is used as an argument. In the above example, every Max Raid Den will spawn a Pokemon with from the pools of Pokemon that can appear in Max Raid Dens in the Ultra Deep Sea biome. The results are the same as if the player was throwing Wishing Pieces into Max Raid Dens in the Ultra Deep Sea biome.
/raidbomb pixelmon:ultra_deep_sea 5
Using the /raidbomb command with a biome and an integer from 1 to 5 as arguments would spawn a Pokemon in every Max Raid Den from the pool for that particular Star Level given for the biome given. In the above example, every Max Raid Den will spawn a Pokemon with from the pool of Star Level 5 spawns that can appear in Max Raid Dens in the Ultra Deep Sea biome.
/raidbomb legendary
Using the /raidbomb command with the string "legendary" as the argument will produce the same results as using Waterdude's Wishing Piece on every nearby Max Raid Den.
/raidbomb clear
Using the /raidbomb command with the string "clear" as the argument will remove every Pokemon from every nearby Max Raid Den, so none of the loaded dens have a Pokemon in them.
A /raidbomb command will make spawning a lot of Pokemon in Raid Dens and removing those Pokemon a lot quicker.