Tents give you somewhere to shelter from bad weather if you're hanging around for a while, somewhere to get changed, somewhere to store and sort all your kit, the ability to cook inside/in your porch, protection from midges, not claustrophobic...and your Laser weighs next to nothing
Bivvy bags are marginally lighter (there isn't much in the weight of 2 bivvy bags v a lightweight 2 man tent though), can be cheaper, don't normally require any pitching and will stand up to as much wind as you will, but you will probably suffer a bit of condensation, get depressed in 12 hours of rain in winter, struggle to cook comfortably if the weather is bad etc etc. Bivvying can either be hugely simple or a massive faff, depending on what the weather is like.
I occasionally bivvy, but that's only because my mountain tent is 4.5kg and not appropriate for some trips. I'm currently eyeing up a 1.7kg 4 season tent which will make the need for bivvying almost obsolete.
That said, there is nothing better than lying in a bivvy on a clear night just staring at the sky