Yes—several types of mobile apps can be very useful alongside a DSLR:

• sun/moon planning apps: Tools like The Photographer’s Ephemeris or similar apps help you plan sunrise, sunset, moonrise, moonset, and shooting direction on a map.
• astronomy / AR sky apps: These can help fine-tune the exact current position and path of the sun or moon when you’re on location.
• long exposure / ND calculators: Handy for quickly converting an unfiltered exposure into the correct shutter time when using ND filters.
• geotagging apps: These log your GPS position while you shoot, so you can later match location data to your photos.
• location scouting apps: Map-based apps with shared photo spots and viewpoints can help when traveling or looking for strong vantage points.
• model release apps: Useful if you photograph people and want paperless model releases in the field.

So there isn’t one single “must-have” app for every photographer, but planning apps, exposure calculators, geotagging, scouting, and release forms are all strong candidates depending on what you shoot.