Viewed practically, curved grading is beneficial because it automatically factors in the difficulty a group of test-takers had with a test. If the majority of students have high (or low) scores then the middling grade will be adjusted there and higher or lower grades awarded based on this performance. In addition, the curve ameliorates the problem of deciding grades that fall very near a grade margin. Clustering of marks establish where the margin should be placed.
However, grading in this way is essentially normative; scores are referenced to the performance of group members. There must always be at least one student who has a lower score than all others, even if that score is quite high when evaluated against specific performance criteria or standards. Conversely, if all students perform poorly relative to a larger population, even the highest graded students may be failing to meet standards. Thus, curved grading makes it difficult to compare groups of students to one another.