Walkthrough tips, Discussion of Campaigns [Heroes 5]

Could you please tell me how to use Astral Gates and what they are for?

Field spell: at the beginning of the turn, you can use it to move a short distance to another area of the map, bypassing some obstacle – for example, to "jump over" a group of monsters or a garrison with a ferocious army...
It consumes 75% of movement points, if I'm not mistaken...