This may seem obvious, but cover allows you to scout for enemies while keeping yourself hidden.
Slide from cover to cover to move across the battlefield until you're at your preferred distance from the enemy.
Cover is also great for lying prone and waiting to ambush unwary opponents,just make sure you aren't surprised yourself from behind.
Make sure you learn the layout of each map, and how to guesstimate enemy movements.
Even the larger maps in Umbrella Corps have obvious routes that tend to be followed,
which in turn should make it easier to predict where enemies are going to come from.
Dealing with enemies in cover
If you're having trouble taking out opponents who are moving up a certain route while staying in cover,
remember there's usually a route that runs in the opposite direction.
Use these routes to get behind the opposing team.
Cover is of course also useful when you're not sure what to do and need to shield yourself from enemy fire;
get behind something and wait for the next opportunity to strike!
See the following articles for more details: