To weaken France's image, to sow chaos, to get security forces focused on other things while they try to get specific targets.
There have been reports about both Russia and Iran being potentially involved.
So far I've seen an article about a Russian getting arrested 2 days ago.
From what I've seen, French newspapers have only mentioned the Russian for now.
Russian spy arrested in Paris after alleged plan to 'destabilize' Olympic Games (Le Monde, English)