The reason is that photography can't always pick up what's important. Sometimes the light is just not right in the field. You can take a photo from all different angles, use the flash to fill-in a shadow, not use the flash to prevent washing out, cast your shadow over a bit... and nothing will get the lighting right. Beautiful days in the field can be too pretty - bright light and hard shadows often make for terrible outcrop photos. There are places where I have three photos for one spot and none of them really show what's important - sometimes the picture is too busy and the interesting bit gets lost in the details. With a sketch you can suggest all those unimportant details and focus on the important ones. Sometimes the feature of interest just isn't very photogenic due to coloring or contrast. Let me show you some examples.
In this example the photo does a pretty good job of capturing the crack fill texture and type of cement. My sketches are ok at it.
In this case I'd say the sketch does a better job of showing the fractures than the photo. (The sketches are not of exactly what's in the picture but similar piece of core.)
This is an outcrop with both standing Calamites fossils and stigmaria (take that young Earth geologists!). The photos don't do it justice, fossils are very faint (although the Calamites do stand out fairly well and look good in close-ups). I have some better examples of problems at the outcrop scale but I didn't sketch them because we're going back to do a detailed description another day.
Obviously when one needs high quality photos or a core or outcrop one can take the time and effort to make sure the photos are good (take core outside, use umbrella to shade, go back to a field site on a cloudy day...) and really show what one wants but for everyday, quick work often a sketch is better. Even with really good photos lines, circles, arrow, and the like are often added to draw the readers eye to the important bits.