UPS is recommended for any computer which doesn't have a second power backup ( Battery) 

Unlike other desktop computers OptiPlex 3080 comes with external power adapter with very little power requirement 65w, 

Sinusoidal UPS is great investment as it will be used to protect other devices too (Printer ETC) . However it's not mandatory even a normal UPS would do the trick 

I have bee using this cheap one for 4 years now and it transfers load without shutting my 500w PSU Desktop.

https://www.numericups.com/products-details?modeldetails=122

Pure Sinewave UPS is fine for all current Dell models.

https://www.amazon.com/CyberPower-CP1500PFCLCD-Sinewave-Outlets-Mini-Tower/dp/B00429N19W/