Because bail doesn’t require that you be charged, it allows the police to disrupt protest by setting conditions while also giving them more time to decide whether they have enough evidence to pursue a charge or for the Crown Prosecution Service to decide what to charge you with.
Even if you are charged, it’s also used more generally because it acts as a punishment in and of itself. Post-charge bail conditions can continue for months and even years, limiting people’s freedoms and adding to other pressures. Bail is used more infrequently now since restrictions on its usage were introduced by the Policing and Crime Act 2017. These restrictions now mean that most protesters are Released Under Investigation (RUI), however many are still bailed. We tend to see a resurgence in the use of bail during multi day protest camps or when there are many protests in a short space of time.