I think a lot of people that jump into kegging early in their brewing life seek to go faster from grain/boil to glass. With kegging and carbonating with CO2 gas (forced carbonation) you can get solid carbonation in under 2-3 weeks (depending on the method you use). Most of the time, you'll get it in 2 weeks if you put it to serving pressure from the start. So you can shave at least a week from your carbonation time, plus several days after that for the chill time (you should chill the bottles for at least 4 days to get the CO2 back into solution, and to get the sediment to compact in the bottle). For some batches, you could be looking at months before it actually carbonates in bottle. Not so with kegging. As long as you don't have any gas side leaks (or leaks in general) you're pretty much assured of carbonation within 2-3 weeks of when you put it on gas.
Personally, I let my brews stay in primary/fermenter/aging tank until they're ready to go to glass (other than needing carbonation). This means I don't need to worry about them needing 'bottle conditioning' time. I was using this same method before I was kegging, so it was an easy thing to keep doing.
IMO, you shouldn't bottle any batch until it's otherwise ready to go to glass. If you need to empty a fermenter to make another batch, and it's not actually ready yet, then you need more fermenters. You should have enough primaries to keep batches where they need to be until ready for next steps.
All that being said, you can also bottle from keg (once carbonated) to allow a batch to bottle age, if you desire. I plan on doing this with some of my big brews that are either in process or soon to be started. My ~12% wee heavy will go to kegs, with some sent to bottles once carbonated. Same with the ~10% old ale I'm making this weekend. I have a big brew on deck that I'm still tweaking that will be in process for several months before it's ready for carbonating. The bottles will be used to take some to other places (friends, and family members) as well as to set some aside so that I can sample later to see how it's aging. How much I bottle from each keg, or if I bottle an entire keg (or half of the batch) has yet to be decided. But, I'm not going to bottle carbonate these (or any others for that matter). With using CO2 to force carbonate a brew, you don't need to worry about it being overcarbonated if you bottle it. Or while in keg for that matter. You can easily adjust the carbonation in the keg both up and down to fit your wishes. IMO, that right there is worth the expense of getting into kegging.
With my methods, I'm essentially (in most simple terms) bulk aging (especially my bigger brews) until I decide they're ready to go to glass. With this method, it gives me more options to tweak a batch before it's going to glass. I could decide that a batch is ready to be moved off of the oak it's currently sitting on. I move it to a fresh vessel and let it sit there for a few more months. I then sample and decide what to do with it. Could be it's ready to go to glass. Could be I want more oak character in it, so I put it on oak again for X months. Once your batch is bottled, you lose all tweaking options with it (other than what you can do in the glass). IMO, this is more important in bigger brews (higher OG/ABV% brews).
For my more moderate OG batches, I find that 3-4 weeks is typically long enough in primary to be great. I then simply rack to my serving kegs (3 gallon kegs), and put one into the brew fridge when a spot opens up. The second keg stays in the wings until I decide to carbonate and serve it up. I do a partial carbonation of the kegs at room temperature. so that they carbonate in the fridge a bit faster/better.
