If you win the Top Scorer challenge your player gets 30 added to their 'finishing' attribute. However, the 30% is added at the start of the new season, so your player has just lost 20%, meaning in reality their 'finishing' attribute goes up by 10%. Still an increase, and if you win it a few seasons in a row with the same player it makes a big difference.

Also, the Top Scorer challenge is only for the league.