I think this sums it up. Taken from http://bible.org/article/anointing-bethany.

This act revealed she knew Christ as:

1. King: Such an extravagant gift was only lavished on a king. This was very appropriate in view of the fact that on the next day He would proclaim Himself the King of Israel through his triumphal entry into Jerusalem.

2. Priest: John 12:3 and Mark 14:8 define this as an anointing. As priests were anointed, this is in keeping with the fact that Christ was a Royal Priest and was about to make atonement for His people.

3. The Savior Who Must Die: As He had told the disciples that He must die, so He must have also told her. While they were unable to grasp this, Mary did. She undoubtedly recognized her sin and need of a suffering Savior and did this as an act of faith and devotion. She understood the reason for His death (her sin), and the significance of His death (her salvation).