Top 8 automatically qualify for the round of 16. 9th-25th enter a playoff round for the other 8 spots.
Round of 16 is seeded so 1st plays 16th 2nd plays 15th and so on so it is beneficial to get in the top 8 as hopefully you'll get an easier draw.
Ive done a few simulations on FM and the 8th place side gets between 14-17 points
Ah cheers for the FM bit. So if you win your first 5 games you might be able to chill for the last 3.
As @WickyWain mentioned I believe there could be pairings to keep 1st and 2nd (possibly more) on the opposite side of the draw in the knockouts. Not convinced that's a big enough incentive to go all out in 8 groups games though.