The exosphere is the outermost layer of Earth's atmosphere. Although it is thin, it absorbs radiation that the sun gives off, shielding the layers of the atmosphere underneath.
The exosphere is found on the very boundaries of Earth's atmosphere. Just underneath the exosphere is the thermosphere, which shrinks and expands according to how much ultraviolet radiation is coming through the exosphere.
According to the National Center for Atmospheric Research, some scientists don't even consider the exosphere a part of Earth's atmosphere. They consider it a part of outer space, as there isn't a distinct upper boundary of the exosphere.
There is a region in the exosphere called the geocorona. This is where the sun's radiation exerts pressure on hydrogen atoms, and these hydrogen atoms scatter ultraviolet radiation and glow as a result. This is a sign that the exosphere has an important role in absorbing radiation and protecting the layers underneath. If the radiation were allowed through the exosphere, it may be harmful to the layers underneath.
As mentioned before, when the thermosphere is exposed to radiation from the sun, it expands. If not for the exosphere providing a boundary for the thermosphere, the thermosphere might never stop expanding, and Earth's atmosphere wouldn't function properly. If Earth's atmosphere doesn't function properly, then neither can Earth's ecosystem.