@j0 It's a way to break up the protest; they can legally arrest someone on a flimsy "suspicion", hold them overnight, then never actually charge them and release them the next day (or even the same evening) - but by that time the protest will have been completely killed off by the arrests.
They usually primarily focus on the most energetic protesters, who 'drive the vibe' so to say. For bonus points, they also get the fingerprints of everybody they've arrested, despite there being no charges.