This structure can be used to make a verb the topic of the conversation, so you can describe it in more detail.
テニスはおもしろいです。
Tennis is fun.
テニスをするのはおもしろいです。
Playing tennis is fun.
テニスを見るのはおもしろいです。
Watching tennis is fun.
The two later examples are more specific in that that they say what the speaker likes to do, rather than just what thing he/she likes.
other adjectives that are often used to describe verbs include むずかしい, たのしい, きけん, たいへん.