G-d informed Moses that one of the purposes of the plague would be to demonstrate to the Egyptians
G-d's mastery over nature.
The Egyptians will recognize that I am G-d.
Shemot 7:5
G-d refers to Himself here by the name Havayah, which indicates how He is not bound by any limitations. One of the implications of this Name, therefore, is that G-d can be merciful; that is, He can exercise kindness even when the laws of logic would demand corrective punishment or vengeance.
In this context, the purpose of the plagues was to introduce the Egyptians to the concept of G-d's mercy. As worshippers of nature, the Egyptians were fully acquainted with G-d's power of apparently blind judgement, by which survival belongs to the fittest. However, the Egyptians did not yet recognize that G-d is greater than nature, and can override the unforgiving laws of nature when He sees fit.
Thus, what later impressed Pharaoh the most was Moses' ability to stop the plagues, rather than to initiate them. The magicians of Pharaoh's court were indeed able to duplicate some of the plagues, for they knew how to manipulate nature to an extent, and were thus able to summon the forces of nature to wreak destruction. But the magicians could not revers or check these forces. Only Moses could do so -- by invoking G-d's mercy through prayer.
We, too, should never doubt G-d's ability or readiness to overrule the natural laws of strict justice, invoking His mercy through prayer.
From Kehot's
Daily Wisdom #2