I seem to bypass it easy with rockets and vehicles. How is it used properly?

Thank you. :)

It stops Tie/Y-wing bombing runs on your base.

Thank you. :)

Often the in-game text (help) isn't as accurate as the manual. For example, the text on a shield generator doesn't mention protection against orbital bombardments. In fact, it's somewhat inaccurate. It says it only prevents lasers from going through it.

You can upgrade it to stop rockets as well, I believe

All you have to do is launch it right outside the shield and the missles go right through it

The rocket went through the sheild because the launcher was within the sheild.
If the sheild is upgraded then no projectiles can past the sheild, form outside.
I did find the Sheild Generator usful against the computer. I can fire out and affect them, but they cannot fire in.

The shield is usefull if the power generator is under the shield which is strangely rare...

I find the shield annoying and it does block damages (u can hear the 'ping' sound of things hiitting the shield). Plus those endless rebel artilery garrison. It just drive yor AT-AT mad. That's why i always go for the powergenerator first( which is always easily reachable.
Bombing run can hit but it's still limited because u can only hit the edge as the enemy base is usually located at the corner.

The power generator is the most important structure as all the defensive buildings depends on it (excpet those on small build pads). U can only pray that the generator is under the shield....

Would be nice if we could move buildings around via galatic map. Expansion request? :)

You can upgrade it to stop rockets as well, I believe

If playing the GC, do you perform this shield upgrade on the galactic map or is it done during the land battle itself?

only during the land battle if it's possible to do that.