In Odoo v13 probabilities on the crm stages have been removed as they did not give much insight into what was happening.
Instead, Odoo v13 uses PLS (predictive lead scoring), which is based on Bayes' theorem.
All in all, you have two flows for your lead probabilities and you can use both at the same time or go from one to the other, ...
You can always use your own manual value that you decide to type. In that case, it will never be erased as it's actually stored in another field.
Or you use the automated flow with the PLS.
Basically, Odoo computes the probability automatically based on historic leads (won/lost) that shared common values (same stage, tags, sales team, country, source, ... depending on what you decide in the CRM settings). From that, Odoo computes the % of winning or losing the deal.
Moreover; whenever an interesting value on the lead is updated, Odoo recomputes the probability of winning the deal or not. (ex: You lose all opportunities based in the UK, which means that setting the UK as the country will decrease the % of winning).
In the beginning, it's hence better not to type anything in the probability field as it allows Bayes to be activated by default on leads (whereas otherwise, you'll need to switch mode through the button as showed in the picture.
If the leads start at 0 in automatic mode, it's indeed because Odoo needs an extra trigger to get started. But since Odoo knows those leads will have to be updated manually anyway (because so far they only have a name), Odoo will be able to trigger the update whenever an interesting field is updated.