Public money, public code
In shortOpen source does not mean free of charge. It means nobody can lock you out.
The demand is simple: software paid for by the public sector should be published under a free licence. What the public funds should belong to the public.
For companies the political side is secondary. The operational side is the interesting one.
What open source means in practice
Nobody can lock you out. A vendor can end support, triple the price, discontinue the product or be acquired. With open source you can keep operating in all four cases, with a different service partner if need be.
Changing your service partner is not changing your product. This is the underrated point. With Nextcloud, Zammad or Odoo you can change who maintains it without touching the application. With a closed product, vendor and product are the same thing.
Auditability. What the software does can be read. For most companies that is theoretical; for security assessments and certifications it is real.
What open source does not mean
Not free of charge. Operations, updates, backup, support and training cost money — with open source often about as much as the licence did before, just at a different point and to a replaceable recipient.
Not automatically European. Keycloak is open source and stewarded by a US corporation; Seafile is open source and comes from Beijing. The licence tells you about your freedom to act, not about the vendor's jurisdiction.
Where it pays off fastest
On the services whose failure stops you immediately and where a grown-up open replacement exists: directory, file storage, ticketing, wiki, video meetings. Precisely the layers where switching is cheapest anyway. See Sovereign workplace.