I recently obtained Lara Croft Go on Steam via my Humble Monthly subscription.
I remembered this game runs on Android platforms.
I happen to have the Steam application installed on my Android.
Can I play the Android version of this game now that I acquired it on Steam?
Generally speaking, no. Steam does not act as DRM or as a platform for mobile games.
Some games may individually choose to grant a mobile copy if you already have a platform copy, but in my experience this traditionally doesn't happen.
In this specific case, Lara Croft GO does not transfer between platforms. Buying a Steam copy will not allow you to play it on your phone, and buying a Play Store or App Store version will not let you play it on your PC.