You can use the compass in a number of ways

1,You can find out if you are on a header or a lift as you'll have taken time before the start to find out how much the wind is shifting. You may have regular veers(shift clockwise) and backs (counter clockwise) over time or there may be a trend to veer or back

2, You may after time, know what your tacking and gybing angles are; the compass can assist you in tacking or gybing at the right time

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