It's a very simple, straightforward and natural way to express filmic ideas. This means that a reader skims through the material, reading only the spoken words, not the description. Dialogue is fast and can keep a story moving. Of course, many stories are more action-dependent and have very little dialogue. Also, dialogue may simply be functional, and not particularly eloquent.
Functional dialogue is in fact, more condusive to filmmaking in general.
If you feel comfortable sketching or doodling, don't hesitate to draw the ideas from your mind. Drawing is another direct way to further shape your ideas on paper. If it is done repetitively, good designs for characters, props, sets, etc. can emerge. Similar to a comic strip, sequences can be drawn to illustrate your conception of a film's scene. I would never recommend drawing an entire storyboard at this early stage, since a storyboard is more of a finished product, prepared as a step before actual filming.