Dip the Karpas? Why?

(1) Karpas: One dips green vegetables in salt water to symbolize that like karpas, we must undergo an entire immersion of one's body for the purpose of a complete purification. One should not be like the person who immerses himself with an impure reptile in his hand, God forbid! This is an allusion to the fact that the Israelites were idolaters while they were in Egypt. When God took them out of Egypt in order sanctify them by giving them the Torah, it was necessary first to remove the impurity that was upon them and to have them perform complete repentance. One immerses the greens to symbolize the Baal Teshuvah (repentant) who is as humble as the plants in the field which are trampled underfoot.
The greens are called karpas, which comes from the word for trampled over; or it is related to the word refes, spelled with a shin instead of a samech, which is the word for mud or earth. (See Rashi in Proverbs, 6) Karpas is a reminder that the Israelites were humiliated by the Egyptians; they were like "mud and dirt."
The combination of the words rehatz and karpas also hint at the process of purification: first cleansing oneself, then immersion and finally 'sprinkling,' as we find in the law of the red heifer: the sprinkling of the ashes of the red heifer mixed in water was done with a bunch of hyssop. (See Nu.19:18) Here we dip a bunch of greens as an allusion to this ritual of purification. Similarly, the people were commanded in Egypt to dip some greens in the blood of the Passover lamb and to place the blood on the lintel and the doorposts of their homes, as it says, You shall take a bunch of hyssops.." (Ex. 12:22)

(1) The Symbolism of Karpas Why was it necessary to tell us that one must make a blessing on the karpas ‘boray peri ha’adamah,’ since we are obligated to make a blessing on any food which we consume? The reason is based on a principle of Halacha: for anything which would normally be improved through cooking, we say the sh’hakol blessing when it is uncooked and the appropriate blessing (in this case ha’adamah) when it is cooked. Based on this the Maharil wonders why, if one uses parsley for the karpas, one would say boray peri ha’adamah? There is a great lesson to be learned from the fact that we say this blessing even though we eat it raw.
This is an allusion to a Midrash: when the Israelite women gave birth in the fields and the Egyptian soldiers would come to kill the children, the ground would swallow up the infants. The Egyptians would then bring oxen to plow up the ground in order to find them. After they left, they broke through the ground and sprouted up like weeds, as it says “I caused thee to multiply as the plants of the field.” (Ezekiel 16:7) In order to remember this great miracle, we eat greens and recite the blessing boray peri ha’adamah even though it is not necessary to recite this blessing under these circumstances.