This pattern marks something as an obvious, expected case, and then adds that another thing is also true. The nuance is like saying 'X goes without saying, and Y too' or 'not only X, but also Y'. It often appears with 'も' later, like 'X はもちろん、Y も...'.

In speech, you'll hear it as a natural way to emphasize one item while adding another. It's not overly formal, but it is more common in writing and structured speech. You can use it with nouns easily: 日本はもちろん、中国でも... (Not only in Japan, but also in China...). For verbs, use the nominalizer の: 食べるのはもちろん、... (Not only eating, but...).

Be careful: it's not the same as もちろん alone, which just means 'of course'. はもちろん specifically sets up an additional point.