An ESPP is a convenient way to buy shares of your company stock, as contributions are often deducted pre-tax directly from payroll. In addition, ESPPs may allow you to buy stock at a discount from the FMV (up to 15%) and/or at a lower price based on a lookback provision (a lookback provision may allow you to purchase stock at the better of the purchase date price or the price on the grant date/offering date of the plan).
The combination of ease of funding and a potentially discounted purchase price make a good ESPP a valuable employee benefit