We need nuclear because closing the gap between 80% and 100% clean energy will be much harder and more expensive without it. Comparing the cost of building energy sources today it may look that nuclear is more expensive than renewables, but that's because we are not counting the cost of the additional infrastructure needed by them, which rises exponentially with the share of renewables in the mix.

Also, no energy source is environmentally neutral. Building renewables and it's supporting infrastructure en masse will have a huge ecological impact. You can reduce that impact using nuclear energy (that also have it's impact on nature, but elsewhere). For example, nuclear power plants kill fish, wind turbines kill birds. By using both energy sources it's easier to limit building plants only in areas, where the impact will be low. We'll be still killing some birds and fish, but the chances that it's well within the ability of populations to recover will be much greater, than if we decided to focus on one source.

We need nuclear because it saves nature.