What would happen if the Earth's axis didn't tilt?
If the Earth's axis didn't tilt we wouldn't have the extremes of seasons .
The Earth's axial tilt is the main reason why we have extremes of seasons outside of the tropics. During the Summer and Winter months one hemisphere is pointing towards the Sun and the other away. This leads to warm Summers and cold Winters.
If there was no tilt, we would effectively have one season which would correspond to Spring or Autumn.
There would be a smaller effect due to the Earth's elliptical orbit. The whole planet would experience an mini Summer in January when Earth is closest to the Sun. It would also experience a mini Winter in July when Earth is furthest from the Sun.