If you've already got emergency savings sufficient for your needs, I agree that you'd be better served by sending that $500 to your student loan(s).
I, personally, house the bulk of my emergency savings in CDs because I'm not planning to touch it and it yields a little better than a vanilla savings account.
To address the comment about liquidity. In addition to my emergency savings I keep plain vanilla savings accounts for miscellaenous sudden expenses. To me "emergency" means lost job, not new water pump for my car; I have other budgeted savings for that but would spend it on a credit card and reimburse myself anyway so liquidity there isn't even that important.
The 18 month CDs I use are barely less liquid than vanilla savings and the penalty is just a couple months of the accrued interest. When you compare a possible early distribution penalty against the years of increased yield you're likely to come out ahead after years of never touching your emergency savings, unless you're budgeted such that a car insurance deductible is an emergency expense.
Emergency funds should be guaranteed and non-volatile. If I lose my job, 90 days of accrued interest isn't a hindrance to breaking open some of my CDs, and the process isn't so daunting that I'd meaningfully harm my finances.
Liquidity in 2017 and liquidity in whatever year a text book was initially written are two totally different animals. My "very illiquid" brokerage account funds are only one transaction and 3 settlement days less liquid than my "very liquid" savings account. There's no call the bank, sell the security, wait for it to clear, my brokerage cuts a check, mail the check, cash the check, etc. I can go from Apple stock on Monday to cash in my hand on like Thursday. On the web portal for the bank that holds my CDs I can instantly transfer the funds from a CD to my checking account there net of a negligible penalty for early distribution. To call CDs illiquid in 2017 is silly.