Resort fees are usually charged to cover amenities like Wi-Fi and gym access, but they're not optional. You have no choice but to pay them even if you choose not to use the amenities they cover.
The majority of resort fees cover perks like free internet services, gym entry and access to the resort pool if there is one. The reason travelers find these fees so frustrating is because they cover perks that should be included in the nightly rate of the hotel.