When in 'high-quality' mode, the laser travels across the entire width of the material at the same speed, but only marks the defined areas. This means that all jobs will be engraved at the same time across the entire engraving surface. The result is a very even, homogeneous laser engraving.
When 'high-quality' mode is not activated, the head will usually travel at a higher speed but only over the width of the area that is being engraved. This means that the material may be engraved deeper in some areas due to the different exposure times. 'High-quality' mode is a more controlled way of lasering.
TLDR: Always activate 'high-quality' mode when a clean and perfect engraving result is desired, and the length of the engraving time is not critical. If you've got the time, why not?