It's Germany, they have labor rights that they want to uphold. This is a so-called "warning strike", to signal that there will be collective legal action if they get fired without abundant severance pay.

Basically TikTok doesn't want to negotiate with the union and the union is showing that there's support for collective legal action instead of a 1-on-1 dismissals that would cost the company way less. The company has an interest in negotiating because it's quite sure to lose the legal battle.