In "Word Focus" Darrell Grim writes:
The use of the expression Alpha and Omega in verses 8 and 11 is significant. These being the first and last letters of the Greek alphabet, the obvious reference is to Christ's eternity. No one else can claim this title, so He uses the emphatic expression “I AM.” Since the expression not only includes the first and the last but everything in between, it also connotes the omnipotence and omniscience of God. In verse 8 this concept is revealed in the word translated the Almighty. The Greek word pantokrator is used nine times in the Apocalypse and only once in the Epistles (2 Cor. 6:18). It is, however, used frequently in the Septuagint and is literally the “all ruler.” It represents the Hebrew word translated as “God of hosts.” He is pictured as the one who commands all and is able to overcome any foe.
Discussion: How does this help you to understand better who Christ is?
Source: Christ, the Triumphant Lord: Adult Teacher's Insights, page 4.