what does getting first seed get? what advantages?

The first placed team in every region gets the first seed. When groups are set for worlds there is only a single first seed in each group.

Essentially you get to dodge every other #1 team from each region until bracket stage.

WTF is first seed?

..First..seed??

Momentum going into the world championship tournament. It's a huge morale boost. You also don't have to worry about facing another #1 seed in the group stage, whether it's EDG, ROX Tigers, or the LMS #1 seed.

It avoids getting matched against what are supposed to be the strongest teams of the other regions.