For example, there are obvious death benefits, etc. but daily type things make a big difference. At our department, if you are injured an cannot work the union organizes people to cover shifts and a meal train to bring food. If you have a child or spouse taken to a hospital in a larger city, they will put $500 in your account that day no questions asked to help cover immediate expenses for lodging and food. They send flowers to funerals for your family members, provide scholarships, pay for training that the department won't, advocate for pay and benefits, etc. They host a large 2 day (so each shift can go) lake party for families and firefighters and more.
Basically, if the union works for its members and shows that it provides value, people will still belong and pay dues. If the union is nervous everyone would quit paying and drop their membership, they need to take a long hard look at what they do for their brothers and sisters and why people no longer want to belong if they do not have to.