1) You can use cover fire as a suppressor, to keep the other team from firing

2) You can use cover fire to keep the other team from moving (this would be for the flankers to use)

3) You can use cover fire to make the other team move out of a key position for your team

When using cover fire make your shots count dont just spray paint hoping to hit someone, you need to be accurate and time you shots, the more accuracy and timing in your shots the more this becomes a tactic not everybody will be on target when giving cover fire, it does not need to hit a certain target but as close to that area as possible.
Try to move in close, when the other team pops his head up set off a few rounds just to keep his head down long enough for the flankers to move into position or to make the other team execute a certain amount of moves
Click here for a little example of what I'm trying to say

When the flankers are in place, the flankers can lay down fire so that you can move up or to make the other team move out and retreat when the other team is retreating do not give chase they might be leading you into a ambush
Cover fire is a strong tactic and hardly looked upon as a tactic most players try to go up gun for gun and eliminate players that way, when if fact if you try it and the other team uses cover fire you're sure to be eliminated as qick as the eye can move.