Mayo Clinic defines chronic sinusitis as swelling and inflammation of nose and head sinuses for three month or longer, despite treatment. This condition interferes with the way mucus normally drains, and makes your nose stuffy
Possible causes are infection, nasal polyps (growths inside the sinuses) or swelling of the lining of the sinuses.

Symptoms 

Common signs: Nasal inflammation ,Thick discolored discharge from the nose (runny nose), Blocked or stuffy nose (congested) causing difficulty breathing from your nose, Pain, tenderness and swelling around your eyes, cheeks, nose or forehead, Reduced sense of smell and taste.

Other signs and symptoms: Ear pain, Headache, Cough or throat clearing, Sore throat 
Otomar benefits

Nasal saline irrigation plays a major role in the treatment of Chronic Sinusitis. It helps thinning the mucus, decreasing swelling and cleaning the hard secretions to prevent infection
Studies have shown that nasal irrigation with isotonic and hypertonic solutions help improve mucociliary clearance time in chronic sinusitis, hence improve patients symptoms 
Research suggests that Hypertonic saline irrigation leads to better symptom control over isotonic solutions in Chronic Sinusitis

Source:
A. Ural et al. Impact of isotonic and hypertonic saline solutions on the mucociliary activity in various nasal pathologies: clinical study.  The journal of Laryngology & Otology (2009), 123, 517-521
E. Succar et al. Nasal saline irrigation: a clinical update. International Forum of Allergy and Rhinology. Vol 9 . No. S 1 , May 2019