From the Super Mario Wiki, the Mario encyclopedia
Wario's Laptop is what Wario uses to set up his microgames in WarioWare, Inc.: Mega Microgame$! The menu and select screens are also made to look like it is on the Game Boy Advance screen. Wario has all of the microgames stored on it and when he unlocks a new one by playing it, it appears on the screen with all of the minigames.
Wario's Laptop re-appears in the intro sequence of Game & Wario with Wario typing on it after calling his friends, and again in the intro of Arrow where it is covered in dust as a reference to its long absence until Wario sneezes the mouse into the laptop, giving him the idea to develop the Arrow game.
Wario's Laptop makes a brief cameo in WarioWare Gold when Wario streams his announcement of the Wario Bowl games.