As for as a general reason to use it over SSM is that it’s free for like under 10k values but it’s not secure by default since it also allows non-encrypted values to be stored. AFAIK you can use basic encrypted values with it the same as SSM, but for more advanced uses there are differences.
Fun fact: SSM is also an acronym for parameter store, it used to be known as EC2 Simple Systems Manager: https://d0.awsstatic.com/whitepapers/whitepaper-ec2-ssm.pdf