Opposition to a European patent is a relatively cheap and quick way to clear the path to market in the face of invalid or dubious patent rights. It is a one-off opportunity to revoke a European patent centrally. It has finality, in that a decision appealed to the EPO Boards of Appeal cannot be appealed further (except, under EPC 2000, in very limited circumstances to the Enlarged Board of Appeal).
No special interest or standing is required on the part of the opponent. There need be no imminent threat or the like.
An opposition can be filed through the anonymity of a "straw man".
A patentee can apply for limitation (and indeed revocation) of his own European patent under the provisions of the EPC 2000, but this is a different procedure and there is no opportunity for a third party to intervene.