It depends where:
In Australia for example, the Red Centre is too hot in summer (and some places are out of bounds), the north is (for most) too wet in wet season.
In Argentina, Patagonia is great in Spring and Autumn, cold and windy in winter, and hot and dry in summer.
In East Africa, it's not a matter of the calendar season but wet and dry seasons.
etc.
For us, defining seasonal conditions is a primary input - when to be where on a given trip. To use Australia as a an example - aim for the Centre in winter, Tasmania in summer (even so it can snow!), and the north in winter, and build the overall route around these conditions. A key driver is also how busy it will be, although you can usually get away from people in most places.