I doubt the water comet idea at L5 would make much sense, due to the Δ-v required to nudge it to L5. But here are the Δ-v costs to get around our immediate neighborhood.
As you can see, it costs the most to get something from Earth to Low Earth Orbit, but there are still lots of reasons that it might be most convenient to schlep something from Earth to Low Earth Orbit rather than use something available from the immediate neighborhood.
Abbreviations
C3 = Escape orbit
GEO = Geosynchronous orbit
GTO = Geostationary transfer orbit
L5 = Earth-Moon fifth Lagrangian point
LEO = Low Earth orbit
As you can see, it costs the most to get something from Earth to Low Earth Orbit, but there are still lots of reasons that it might be most convenient to schlep something from Earth to Low Earth Orbit rather than use something available from the immediate neighborhood.
Abbreviations
C3 = Escape orbit
GEO = Geosynchronous orbit
GTO = Geostationary transfer orbit
L5 = Earth-Moon fifth Lagrangian point
LEO = Low Earth orbit
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