There is no real answer to the question as to whether or not you need to add a shaving bowl to your routine. In short, if you are using a hard soap then not really, if you prefer using a cream then a shaving bowl may prove more useful.
If you are using a hard soap, you could use a couple of methods to build your lather other than with a bowl, either lathering directly on your face or using the palm of your hand. Whether or not to use a shaving bowl is really a matter of preference – some do & some don’t. I find that face lathering uses more soap and takes me more time than with a shave bowl, while I just don’t enjoy using the palm of my hand to build lather. Personally I find using a bowl helps me build up a nice big lather.
On the other hand, if you prefer to use shave creams rather than hard soap you may find using a shave bowl or mug useful. By using a bowl you can dictate the amount of water you are adding to your soap, adding more as you need to. You want to avoid lathering your cream in its tub because you will use more cream than you need to, and building a lather that is not runny is a bit more difficult.
Of course there is no need to out and purchase a mug specifically designed for shaving, you probably have something in your house that will perform the task just as well. I find that a mug or a bowl with a larger opening works better than one with a small mouth.