We don't really know. But if life started through some extremely lucky event like lightning repeatedly hitting just the right collection of particles in an early planet's ocean, then panspermia gives you a plausible mechanism how this happening just once in a galaxy could still lead to multiple planets having life (and maybe one of them developing into intelligent life that can ask itself how this happened). It opens up the number of possible timelines for the development of life because it allows our evolutionary heritage to be older than our planet alone allows for.