The Raiders made their pick at the top of the draft. When the selection came up, the guy they wanted to upgrade their pass rush long term was sitting there looking at him. The Raiders don't use a 3-4 base defense but they definitely utilize the 3-4 in certain situations. Now they have a pass rusher who can execute those duties.
Dennis Allen was the defensive coordinator in Denver for one season before the Raiders nabbed him as head coach. In Denver, he had one of the best pass rushers in the game - Von Miller. Miller can line up as an outside linebacker or defensive end in a 4-3 and is arguably the best 3-4 outside linebacker in the game. Dennis Allen sees that kind of player in Khalil Mack.
"He'll have his hand on the ground some, too. I envision his role being very similar to what we did with Von Miller," said Dennis Allen. "I saw a lot of similarities between him and Von Miller. And the thing that really was attractive about Khalil Mack was the fact that he understands how to rush the passer. And he understands how to rush the passer with power. And to be successful in the National Football League in getting after the passer, I've made that statement several times about everything in the National Football League is about affecting the passer. Whether it be offensively with weapons, or whether it be defensively, getting after the passer. He understands how to rush the passer, and yeah, I see a lot of similarities between he and Von Miller."
His role on the team utilizing his versatility should be similar to what Sio Moore does for the Raiders. He was the team's third round pick last season and was named to the All-Rookie team after a standout first season in which he had 49 tackles (38 solo) with 4.5 sacks from the strong side linebacker position.
With both Moore and Mack in the linebacking corps, the Raiders could play a lot more 3-4 sets and perhaps even shift to a 3-4 base altogether. Versatility is the key. Pass rush is also the key. And that's what Mack does.