Why timing it before, not after, changes things
A coating applied before a trip means every bug strike, bit of tar and layer of dust picked up along the way sits on top of the coating rather than bonding directly to the panel. That's the entire practical benefit — it doesn't stop grime from landing, but it stops it from sticking hard.
Applied after the trip, a coating is still worth doing, but you're sealing in whatever's already there rather than getting the benefit while it matters most — on the road.
What it actually does once you're travelling
A coated panel wipes down with a lot less effort at a rest stop or caravan park — bug residue and road grime sit on the surface instead of gripping it, so a quick wipe does what used to need a proper scrub.
It also buys time. If a proper wash isn't possible until you're home, a coated panel handles that delay far better than bare paint or gel-coat does.
Booking it in before you leave
Coating needs a clean, dry panel to bond to properly, so it's not a same-day, last-minute job — it's worth booking a wash-and-coat combination a week or two out from departure, not the night before you hitch up.