What would happen if the tilt of the earth was 0 degrees?
The days and nights would be about the same length and there would be no seasons.
The axial tilt causes the days to be longer than the nights in Summer and shorter in Winter. It also causes the seasons as one hemisphere gets more sunlight during its Summer and less during its Winter.
It the tilt angle was zero, then the days and nights would stay at the same length and there would be no seasons.
The higher latitudes would not get the extremes of weather that they get now. It would have a big impact on migratory animals as there would be no need to migrate.