BPM and initial key are used heavily by DJ's and others who need to consider how music "builds" over time.
A few examples:
generally DJ's want BPM (tempo) to build over time so you don't go from a "fast" song to a bit slower one...even a slight drop in tempo can cause people to leave a dance floor. DJ's doing beat mixing use BPM to find songs of similar BPMs that may then mix together well. The tonal "key" of a song can also help with mixing songs together or to create smooth transitions. People exercising or running often find they like a certain tempo for running, another for cool down, etc.Here's an article to learn more:
The Best Running Song BPM (and How to Find Your Running BPM)
https://www.beachbodyondemand.com/blog/find-running-bpm