What is the button battery on the motherboard for?

I got my Thunderbolt wet reinstalling the screen protector 2 years ago. The front camera stopped working that day. Recently the wifi has stopped working too. It turns on and off by itself regardless of what I do. I disassembled my phone today to clean the motherboard and look for any corrosion. While examining the board I noticed what appears to be a tiny button battery on the motherboard on the top upper side of the board just left of the upper volume button. What is the purpose of this battery?

Update. I cleaned the motherboard and ribbon cables with alcohol but still no wifi or front camera function. Plus side is that I didn't damage it. Thanks for the service docs.