- Cash accounts do not allow short positions, except for covered calls or puts
- Day-trading has different restrictions in the two account types
- Proceeds from sales are available immediately in margin accounts
- Margin will allow you to skate the delay between the start of and credit for a deposit
- A margin account is like any other line of credit, and part of your credit report
- Margin accounts can carry substantially more risk
In long form:
Spreads and shorts are not allowed in cash accounts, except for covered options. Brokers will allow clients to roll option positions in a single transaction, which look like spreads, but these are not actually "sell to open" transactions. "Sell to open" is forbidden in cash accounts. Short positions from closing the long half of a covered trade are verboten.
Day-trading is allowed in both margin and cash accounts. However, "pattern day-trading" only applies to margin accounts, and requires a minimum account balance of $25,000. Cash accounts are free to buy and sell the same security on the same day over and over, provided that there is sufficient buying power to pay for opening a new position. Since proceeds are held for both stock and option sales in a cash account, that means buying power available at the start of the day will drop with each purchase and not rise again until settlement.
Unsettled funds are available immediately within margin accounts, without restriction. In cash accounts, using unsettled funds to purchase securities will require you to hold the new position until funds settle -- otherwise your account will be blocked for "free-riding".
Legally, you can buy securities in a cash account without available cash on deposit with the broker, but most brokers don't allow this, and some will aggressively liquidate any position that you are somehow able to enter for which you didn't have available cash already on deposit. In a margin account, margin can help gloss over the few days between purchase and deposit, allowing you to be somewhat more aggressive in investing funds.
A margin account will allow you to make an investment if you feel the opportunity is right before requiring you to deposit the funds. See a great opportunity? With sufficient margin, you can open the trade immediately and then run to the bank to deposit funds, rather than being stuck waiting for funds to be credited to your account.
Margin accounts might show up on your credit report.
The possibility of losing more than you invested, having positions liquidated when you least expect it, your broker doing possibly stupid things in order to close out an over-margined account, and other consequences are all very serious risks of margin accounts. Although you mentioned awareness of this issue, any answer is not complete with mentioning those risks.