The advantages of fixed ip addresses are technical facts.
My question is: Why is this question closed as "primarily opinion-based"?
Yes it would use facts, but deciding which facts to put forth as the advantage or disadvantage is based on personal preference and opinion.
One person who thinks being identified with the same IP across sessions and sites is a good thing. For someone else they would rather the IP change as often as the hour.
What is the actual issue here?
Q: I can't decide if I should pay more for a fixed IP address
A: Why do you want a fixed IP address?
Q: I don't know
These questions are based on the fear of missing out. That unless you read yet another page listing technical details and have another set of people tell you, you might miss out on something because you still can't decide why you're looking at it.