Dear Arihjain,
Yes, might be a proxy to understand the downloads but not cent percent accurate because of two scenarios based on my understanding:
1. If the Application was downloaded but not opened at all, First Launch will not be counted.
2. When Application Cache was cleared and opened again, First Launch will fire however the download didn't happen at all.
Both the above scenarios didn't contribute much however just want to add.
Thank You, Arun.