The RNA-world hypothesis conjectures that life started via a chemical soup developing things similar to viroids.
These early RNA-based replicators would go on to become more complicated, first developing a protective barrier and then specialized organelles.
As a byproduct of that process, viruses developed which could interact with organelles (or just the soup inside a cell) to reproduce themselves without being part of the host "genetic code".
In approximate complexity order:
- self-replicating compound, in solution with precursors
- viroid
- virus
- bacteria
- archaea
- eukaryota
From that perspective, hiding viroids (or similar simple replicators) in comets and boosting them into other systems would be a relatively efficient way to "seed" life around the cosmos.