Well, there are a couple reasons.
In the most simplistic example, let's say you've made one small (1 hr) container and you begin to process it at the Research Center. It's going to take 1 hour to process, so you refill your Omega Lab to fill another container. Uh oh, you get an 8 hour container. Now your Research Center is going to sit empty for 7 hours after it finishes your small container until that next container is ready in the Omega Lab. If you have additional Omega Labs, you have a better chance of having a new container to process any time you get an open slot in your Research Center.
Even when you have all 3 slots full in the Research Center, it's good to have additional containers standing by so you can get the timing of completing them more efficient by coordinating the size you process vs. what's waiting in the queue.
Beyond 3 Omega Labs is really not needed, IMO, since there are only 3 slots in the Research Center and you are limited to only one Research Center, but if you have 4, you are pretty much guaranteed to have a container ready no matter how many small vs. large you may get.
Also, Contest of Mayors sometimes has an assignment which calls for processing and collecting Omega containers. Multiple Omega Labs helps complete that task quicker as you can simultaneously be making them. This way you increase your odds of getting small containers to process so you can complete the assignment quicker.
Hope that helps.
