[rally]

Apart from the ease and convenience of not having to setup every night, the really big plus factor is avoiding the risk factor and making yourself more productive.

To explain
If you check the weather and it looks iffy - you will not bother to setup because the likelihood of success is poor, but the problem is our assessment of weather isn't good - we cant tell by looking at the local skies if its going to be good or bad in another hour let alone 3 or 8 hours. Nor can the BOM.
So you tend not to set up as often as you should to avoid the downside of getting your gear soaked or simply wasting an entire evening - night after night !

By having an Obs you can just take advantage of the skies as and when they appear without the effort of setup and dismantly, without worrying that you may only get 1 hour of useful observing time

If your interest is astrophotography then the considerable extra time it takes to get everything working properly (eg perfect polar alignments, drift alignment etc adds heavily to the risk assessment if you dont have an Obs
Plus many things you have to do in that field require lots of testing and since you will never get chance to do a lot of that without an obs you can't and don't ever advance.
It can take up to three hours to setup and dismantle some rigs.

I am guessing that having an obs even for a seasoned observer who will setup at every possible opportunity is doubled - for a less seasoned person its probably a factor of 10 times

Some other pros

Polar alignment stays aligned from one night to the next
Testing gear that needs some constants over a long period is now easy
You can refine things over time
A place to store all your gear rather than the lounge room, dining room table etc and keep peace.
You can buy things that nobody needs to know about because they are now well and truly out of sight !
Less setups means less chance of breakage,
Improves your back not having to lug everything around !
Threads, and cables dont get damaged with repeated use
You wil observe on nights you never could before because you can take advantage of a window of opportunity that is less than the normal set up and dismantle time - eg after you have come home from a late night out.
A place to work on your gear
Since everything is setup and running - you can't forget something or leave it at home and ruin the night !
No need for battery powered equipment anymore - mains power can be used.

Then there is all the Obs specific physical things
Its warmer and less windy in an Obs - so you are more comfortable
Less wind means better OTA stability - critical for imaging
Creature comforts can be included - fridge, kettle, (heater !?) desktop computer and proper keyboard and mouse
Tables, storage units etc rather than working off the wet lawn or a portable picnic table
Use a pier instead of a tripod for better stability
Keep the noise away from the house when others inside are sleeping

Of course if you are just using a Dob and not a GEM then not all of this applies (to the same extent) as the convenience of using a Dob for visual is the ability to setup and packup very quickly.

Theres more !