The main goal in Omaha Hi Lo is to win the whole pot, getting half often puts you barely ahead of where you were (particularly when there are fewer players).
For example, say in a 4 player game where everyone bets £10 each. The pot totals £40, you win the low hand your share is £20 (£10 profit). Had you 'scooped the pot' (£40) you would have made £30. Instead of making 100% profit you would get 300% profit in this case.
Since you can use a different combination of your available cards on each hand, you can win the entire pot by having the best high and low hand. In situations where no player qualifies for the low hand the winner of the high hand takes all.