While it's not mandatory to have a guide for trekking in Nepal (except in some restricted areas), hiring one is highly recommended, especially if you're new to trekking or exploring high-altitude trails. A guide ensures your safety, helps navigate challenging routes, and provides insights into local culture and traditions. They can also assist in emergencies, manage logistics, and communicate with locals, which can be invaluable in remote areas.
If you're trekking well-marked trails like the Annapurna or Everest Base Camp, you could go solo, but having a guide enhances the experience and offers peace of mind. For off-the-beaten-path treks, a guide is almost essential.