My current campaign has been planned out completely, using existing adventures strung up from level 1 to 20. Of course, they require the necessary adjustment and there is always room for improvisation, but I tend to like the idea of where I'm going, because it allows me to create a more believable world.
To give a few examples, if you want to play a certain adventure in the future, it might be possible to introduce a small part of the plot line or an NPC long before you play it. If the PCs befriend an NPC who needs their help a couple of levels later, they will be much more willing to help. Also, I planned my campaign within the confines of one country. Knowing that I'm going to use a certain location in future for a certain adventure, allows me to already create the correct setting if the PCs pass the location before the adventure. The inn is already the inn I need in the future adventure, the priest who is featured in it, already has his temple there, the statue on the market place, which will be the focal point of the adventure, is already in place etc.
So I think planning in advance really enhances the story line and believability of the world. It is without a doubt the only way to create a story arc that goes from level 1 to level 20 that will remain somewhat coherent throughout the campaign. And of course, using published adventures, I retain my own flexibility. I've not spent hours of preparation on the adventures, so if the need arises to make (even major) adjustments, I won't hesitate to do so.