It emphasizes a contrast between two statements, highlighting that something is true despite a previous statement.
Here are some examples:
* French: J'aime le chocolat, mais seulement le noir.
* English: I like chocolate, but only the dark kind.
* French: Elle est riche, mais seulement en papier.
* English: She is rich, but only on paper.
The phrase emphasizes that the second statement is a limitation or qualification of the first.