The actual goal of those border controls is just populist knee jerk bullshit to score sympathies with the people who are increasingly voting for the Nazi party.
The political establishment has been predictably unsuccessfully trying to appease the far right by copying their talking points for years by now. This is just another escalation of this disgraceful act, after the Nazis scored high in two state elections.
Yeah, everything has one reason. It's not as if another Schengen country is lax on their border and cozy with Russia and china while the geopolitical tensions, espionage and sabotage as high also.
But knee jerk border checks on a handful roads won't change that in any meaningful way. All it will do is funnel away resources that would be needed elsewhere more urgently. And look impressive on the news.
All while undermining one of the greatest achievements of the EU.