And what's the benefit of him (or Boris) going?
What's the benefit? This is a naive question, I'm sorry...
The aim is to preserve the workings of our democracy.
He has broken the code of conduct that advisors are supposed to uphold. Go and read them, they're publicly available to read.
When someone breaks the rules, there are consequences to hand out. If not, then rules (which are there for the good of your democracy) are useless.
Boris Johnson is completely free to hire another advisor who hasn't broken the rules or humiliated the government.
If he'd come out, said sorry, and that he'd pay the fine if Durham police deemed that one was due then this would be over.
As it is, they've all showed what thoroughly deceitful liars they are, people are angry, and it's festering like an unflushed turd.