I made small test app which shows daily steps using Google Fit API. During testing, I was not getting any steps count. But later I realised that my Samsung S5 doesn't come's with Google Fit app so I downloaded it and after feeding some data then I tested my app again and Now I can see my steps count on my test app.
So Does this means that in order to use Google Fit API all my app users must have installed Google Fit App?
While the Google Fit app does automatically collect some information for you (such as steps, current activity) throughout the day, it should not be assumed that it is installed. In fact, for any kind of data you wish to show in your app using the Google Fit API, you should use the Recording API to record the data yourself.
In the background, the Google Fit API will dedup any identical requests across multiple apps (i.e., if your app and the Google Fit app are both installed and both record steps, it won't cost double the battery life).