The barrier fence is not built to prevent individual illegal border crossings, but to prevent instrumentalised or other illegal migration. Even though the barrier fence improves detection of individual illegal border crossings and arresting the said persons, it will not prevent all of them.
The barrier fence will be built on areas where the instrumentalised migration is likely be greatest. There are widespread wilderness areas along the 1,340 kilometres long eastern border of Finland, especially on the Russian side, where widespread instrumentalised migration is so unlikely that it is not sensible to build a fence on these areas.