Since I know which position I'm drafting in, I use it to gain some insight. Granted, no one mock draft is going to duplicate how the picks go in your draft, but it still gives you a general idea.
1. Positional Picks: If I go RB-RB-WR-WR, do I like the result? How about WR-RB-RB? How about QB-TE-RB?
2. Need-To-Have Sleepers: If I really need to have someone who I think is going to have a breakout year, I want to see how high this person is being drafted, and then draft them a round earlier in case anyone else wants the same guy. For example, I NEED to have Cordarrelle Patterson this year, I think he's going to have a Josh Gordon type breakout year. I also really want to pick up Carlos Hyde in the later rounds since I am in a keeper league and I think he will be the 9ers starting running back in the near future. It's nice to know when a good time to pick them up would be to guarantee they end up on my team.
On the flip side, if I don't know which position I'm drafting in, mock drafts allow you to pick any position you want and get a good feel for the same two points mentioned above. My draft strategy would probably be different if I'm drafting #1 vs. #10.
Plus, I like to engage in conversation with the other drafters to hear their takes on certain guys or draft styles.