Why do Ripple (XRP) and Stellar (XLM) require a minimum balance reserve to initiate a transaction?

Why do Ripple (XRP) and Stellar (XLM) require a minimum balance reserve to initiate a transaction?

Ripple (XRP) and Stellar (XLM) assets are unique in that, unlike other assets, they both enforce reserve requirements in order to protect their shared global ledgers from growing excessively large as the result of spam or malicious usage.

These networks require minimum balances on accounts in order to prevent people from overwhelming the network and to aid in prioritization.

To submit transactions, an address must hold a minimum amount of XLM or XRP. You cannot send these reserved amounts to other addresses. To activate a new address, you must send enough XLM or XRP to meet the minimum reserve requirements.

Current minimum reserve requirements:
Stellar (XLM) is currently 1 XLM
Ripple (XRP) is currently 10 XRP