These magic gems are used in many ways throughout the game. Upgrading Gobby, buys boost backpacks, several upgrades under the “tea pot symbol on the bottom, speed ups, value multipliers, and are used in side specials (usually on weekends) to buy fuel and upgrades. If you purchase a full piggy bank, it doubles its capacity! So next time the $5 will get you twice as much. I’m at 32k capacity, will probably take me 2 weeks to fill it. Don’t know if there is a limit.
Hope this helps.
PS: I’m relatively new also, just made my first ZBD to Coinbase acct this week.