It's not just a spaceship, it's a transformer!

Versatile Viking
Predator and bird of prey
Transform and roll out

The Terran race in Starcraft seems to have a theme of versatility, and of being able to specialize things in multiple ways. The siege tank is the epitome of this mentality, being able to take the form of a standard tank, a static defensive turret, and a offensive siege weapon. But the theme is present in many ways: command centers can become orbital commands or planetary fortresses, orbital commands can call in either extra supplies or reconnaissance or minerals or repairing, and all production buildings can specialize in making lots of basic units, or the more advanced units.

And of course this theme shows up in the Viking. In the air, a potent air superiority fighter, with huge range and good speed. On the ground, a... ground unit, that can... walk around, shoot things occasionally, and blow up spectacularly. Okay, so the theme is not shown very well here. I understand that Blizzard wanted to give Terrans some much-needed anti-air, but make it not too good against ground (a role the Banshee fills very well). I just think they made it a little TOO not-good on the ground. As cool as a transforming mecha spaceship robot is, the motto of any Viking pilot should be, "If you need to land this bird, you've already lost." Sad, but true!