I believe that indirect chemical evidence (isotope concentrations in Australian zircons) suggests that life may have existed on Earth as far back as 4.1 billion years. That's the 'hellish' Hadean eon in which the surface of the Earth was still cooling and recovering from the planetary impact with a Mars-sized body that is hypothesized to have created the Moon. In other words, the Earth might not have had a solid surface stable enough for life to form for much of the time previous to life's first appearance.
https://en.wikipedia.org/wiki/Hadean
https://en.wikipedia.org/wiki/Giant_impact_hypothesis
This new evidence would put the origin of life on Earth prior to the Late Heavy Bombardment that pocked the Moon with its craters and probably involved tens of thousands of 'planet killer' asteroid impacts on Earth. So, if the evidence for the early appearance of life I just mentioned up above is true, either the earliest life would have had to have somehow survived the LHB, or else life will have had to have originated on Earth more than once.
https://en.wikipedia.org/wiki/Late_Heavy_Bombardment
The point is that if life seemingly appeared on Earth very early in its history, in an eon in which the Earth's crust was newly solidified and when the Earth was still subject to constant asteroid bombardment, then there would have been little opportunity for prebiotic evolution of simpler chemical replicators to result in the origin of the first procaryotic cell. The alternative to a lengthy prebiotic process would seem to be the idea that bacterial life originated all-at-once, by some hugely improbable leap from a bunch of chemicals in a hypothetical "primordial soup" to a fully-formed cell, complete with its molecular genetics, chemical metabolism, cellular anatomy and everything else. That's like blowing up a pile of building materials with a bomb and expecting the explosion to produce a house. (Bacteria are vastly more complex than houses.) It might not be absolutely impossible, but the probabilities would be so low as to effectively be the same thing.
Addy Pross says in his '
What is Life - How Chemistry Becomes Biology' (2012, Oxford U. Press) p. xi, "The simple truth is that the most basic living system, a bacterial cell, is a highly organized far-from-equilibrium functional system, which in a thermodynamic sense mimics the operation of a refrigerator, but is orders of magnitude more complex. The refrigerator involves the cooperative interaction of at most several dozen components, whereas a bacterial cell involves the interaction of thousands of different molecules and molecular aggregates, some of enormous complexity themselves, all within a network of thousands of synchronized chemical reactions."
Moving the origin of life elsewhere, earlier in the universe's history, supplies time, time in which the origin of the first cells could have happened more incrementally, step-by-step, through evolution operating on pre-biological chemical replicators. That's the advantage of panspermia. The disadvantage is explaining how life made its way here.