We do not need Euratom.
We use Euratom for nuclear fuel, and cancer treatment.
Result: Bad
The European Atomic Energy Community (EAEC) is responsible for developing and distributing nuclear energy, while selling surplus to non-member states.
Switzerland participates as an "associated state", but the UK currently intends to leave.
This may be a problem when it comes to obtaining the radioactive isotopes needed for radiotherapy (typically used in Cancer treatment), and generating electricity.
They are also working on the International Fusion Reactor - which should be considerably safer than our current Fission reactors.
If you disagree, or would like to add more details, please let me know.
You can suggest a reason for leaving the EU, or look at the other reasons.