Sure, it can go up in value along with the market. However, if they could put it to use while still retaining it, that would provide a much larger financial benefit.

I totally agree with you here however I wonder how best Bitcoin can be put to use by a company like Tesla. For security reasons, they wouldn't want to lend it out to exchanges through staking. The nature of Bitcoin doesn't fit in well when it comes to DEFI. The reward from few exchanges that offers earning opportunity with Bitcoin is not encouraging except for the fact that Tesla might have something tangible in the end due to the amount they posseses.
I agree with you, idle assets doesn't promote growth.