Back to basics.
When entering a country one is granted leave to enter.
Because some countries tend to have nationals who tend to be a bit of a nuisance (overstaying or slaughtering the population of the host country) their nationals require pre-clearence or a 'visa' (for historic reasons this is called an Entry Clearence by UK authorities in relation to Commonwealth countries but it's the same thing).
This means that checks on the punters background can be done before he/she sets off, by the visa/entry certificate officer rather than the Border Farce officer holding them in the SEA (secondary examination area) on arrival while employment history, bank balance,education documents etc care checked out.
As of today the different countries of the EU have different visa requirements/entry criteria for different countries.
The Etias system would standardise processing of ALL non EU nationals into the EU as explained here:-
Plus border control would be speeded up for those with a visa-waiver and ePassports (see https://en.wikipedia.org/wiki/EPassport_gates)
which are being rolled out as the international standard of the MRP (machine readable passport) of old.
If you talk to anyone in-the-loop on these matters cross border information between ourselves and the EU on immigration/security matters will continue, especially with Germany/France who we almost trust (though not as much as we trust our colleagues in the Five Eyes! https://www.privacyinternational.org/node/51
) who will be able to disseminate information on a need to know basis to the more leaky members of the EU which means we can ALL keep an eye on the bad guys we ALL agree are bad guys.
One of the more interesting questions we should be asking is why HMG does not emulate the system when dealing with Immigration Control.
Since we already charge for a visa/entry certificate it is quite a nice little earner when you charge people to avail themselves of a visa waiver to save themselves being charged for a visa (neither of which guarantees entry anyway)!
'Be polite, be professional, but always have a plan to kill everyone you meet' General James 'Mad Dog' Mattis.