Ever heard of the saying "learn to walk before you run"?
Better prepare for the run before you start the journey.
As in a vacation, theres three parts, getting to the destination, staying there and returning home. Of course you have to get there before you leave there, but you don't plan your arrival and not plan your stay and departure. Nor do you plan your arrival and stay, then plan your departure sometime after you arrive. Planning all ahead allows possible bumps and difficulties in your journey to be acknowledged and prepared for. If you arrive without booking a hotel, and find out they're all booked, rather find that out at home than in a foreign land. And if you made a budget for the trip, planned on renting a compact car when you arrived but find them rented out, then you would have to go over your budget for a larger car. Could've been avoided by planning, go to a different car rental company, change your budget at home etc. Planning saves money and time.