The last time I checked, the intent of the Imperials was to destroy the Rebel Alliance, and they almost did. Hoth's shield was unique in the fact that it dissipated energy directly into the planet's interior, therefore negating any damage they could've done... At least that's what they say, eh?
In any case, they wanted it to be a quick, decisive strike, yet they were detected coming in, therefore foiling their plans and saving the Alliance.

[i]By the way, if you were wondering, I'm definitely an Imperial.[/i] Fuck the Sith though, I just wanna be cool and shoot Rebel scum. ALSO IT'S A MYTH THAT STORMTROOPERS CAN'T AIM FOR SHIT [b]triggered[/b]