The Wizard Tower is an iconic adventure location. It is my favorite because it allows the most variety when it comes to adventure design. When I am designing an adventure for my groups, I want to keep throwing new things at them. Kicking in doors, killing orcs and taking their treasure is fun, and I certainly make sure to include those sorts of things, but I also like to include puzzles, traps and different sorts of challenges. A Wizard Tower allows me to break the "normal" rules of dungeon design and throw oddball or the unexpected into the mix.
Everything can be explained with "it's magic" no matter how unusual it is. If I want an impassable wall that can only be bypassed by solving an intelligence puzzle, it is impassable because of magic. If I want an 8-headed Hydra to appear out of a small orb that falls to the ground and breaks after the players fail to correctly navigate a trap, it works because of magic. The magic of the Wizard allows me to do just about anything. I can present my players with a wide variety of challenges while still retaining some level of plausibility.