(First of all I'm no lawyer and this subject touches contract law in Germany.
I think in Germany there is something like contract through consistent action.
This is a specific german legal term and I don't think that the translation covers the same things in english.
So if a company payed a bonus 5 years in a row and in the sixth year doesn't you might have grounds for a lawsuit to get the bonus in the sixth year, too since the company created a contract in the 5 years before by paying the bonus.
With this phrase in your contract the company protects itself against the contract through action by clarifying their actions in a written contract.
The alternative to this snippet in your contract probably would be no bonus.
Edit: Before an edit there was a mention of a standart phrase that I have seen a couple of times if a contract has a bonus.
Anyway the bonus should be mentioned in the contract. I wouldn't sign the contract how you paint it in your current edit
But don't be surprised if they keep this block to protect themselves
,,Die Zahlung etwaiger weiterer Gratifikationen, Tantiemen, Prämien
oder sonstige Leistungen liegt im freien Ermessen der Gesellschaft
un begründet kein Rechtanspruch für die Zükunft auch wenn die
Gesellschaft solche Leistungen wiederholt und ohne ausdrücklichen
Vorbehlat der Freiwilligkeit leistet(Ausschluss der betreiblichen
(translation in his edits)