[Kolano]

You can obtain most of the same things via customizations that you can gain from cooking a ROM via the kitchen, or using someone else's custom ROM. However, some advantages of doing so include:
-Freeing phone storage memory. For instance, after flashing my phone has all the software I am interested in having, but is only using 8.96m of storage memory. Admittedly with today's larger internal memories, and add-on cards this may not be a big deal.
-Allowing settings to not be lost by a hard reset. Particularly if you mess around with your phone a lot, I've found it easy to get it into states requiring a hard reset, which is painful when it removes all your phones customizations. This is more true of cooking your own ROM though, as with custom ROMs your stuck with whatever the author has setup.
-Allowing adjustments to some settings such as page pool size that can't be customized without flashing.