If you are bottle carbonating, you need to mix in priming solution (more sugar) so that your yeast can carb up the beer once it's bottled. One reason to use a bottling bucket is you can add the priming solution to it, then siphon your beer on top of it to get it evenly mixed.
If you are carb-tabbing or adding pre-measured amounts of priming solution to the bottles (note: not only is this more work but it is much more difficult to get even carbonation) you could hypothetically skip the bottling bucket, but you will need to find some other way to gravity feed to your bottles. If you have a bottling bucket with a spigot at the bottom, your gravity feed problems are solved already.
Also, they're cheap and easy to use, so provided you have the space for another bucket it's a worthwhile investment.