Money Laundering: Having a legitimate business allows Grissom's accountants to say that his ill-gotten gains are actually from his business. It explains his affluence and since he pays taxes, the Feds don't have the tax evasion route to take him down.
No-show jobs: Grissom can give his criminal underlings "jobs" at the plant where they do not actually have to show up. This basically launders their salaries as criminals and makes them have the facade of legitimate citizens. It also checks a box for those out on parole who have to have a job in order to stay out of prison.
Equipment and facilities: The equipment and facilities of the plant are at Grissom's disposal for criminal endeavors, if he needs them. This could include trucks for smuggling, warehouse space for storing stolen goods, secure buildings for meetings or for stashing hostages, etc.
Chemicals, specifically, have lots of potential uses for criminals: Just watch Breaking Bad some time. Chemicals made at the plant can be precursors for drug manufacturing, they can be used to destroy evidence, they can be used to dispose of bodies, they can be used to blow stuff up, and they can be used to murder people (which Joker does, later in the film).