Tibet and Nepal are located in the hills and foothills of the Himalayas, the natural scenery is different. Tibet has a world-class glacier, lakes and other cold landscape resources, while the territory of Nepal is a green-clad, beautiful scenery. In addition, the travel time between the two places is highly complementary. Nepal's golden season is in November to next March, in contrast, the Tibetan golden season between April and October.
Tibet and Nepal have a unique and complex culture, covering from lifestyle, religion, architecture, festivals and other aspects. So visitors interested in Tibet may also be interested in Nepal, plus two places in the geographical location are also very close, so to visit two places is a wise choice. And the only international flight to Lhasa is from Kathmandu, which also facilitates transportation between Tibet and Nepal. Or you may take overland tour.
Note: If you are going to enter Tibet from Kathmandu, you will be required to apply for your China Group Visa instead of China Visa. Getting China Group Visa usually takes 3 working days. So please leave a few extra days in Nepal.