When brakes need service, a fair question is whether you can just swap the pads and leave the rotors alone to save money. Sometimes yes - but it depends on the rotor condition. Here is when a pads-only job works.
When pads-only is fine
If the rotors are still above minimum thickness, smooth, and not warped or heavily grooved, you can install fresh pads on the existing rotors. Measure the rotor thickness and check for scoring first - see rotor minimum thickness. Good rotors plus new pads is a legitimate, common repair.
When the rotors need attention
If the rotors are grooved, warped, or near minimum thickness, new pads alone will not fix a pulsation and will not bed well against a bad surface; see resurface vs replace. Pads run down to metal usually score the rotors; see why brakes grind. Then rotors must be resurfaced or replaced.
Always do both sides
Whether pads-only or pads-and-rotors, service both wheels on an axle together for even braking - see replace in pairs. Never do just one corner.
Do it right
Inspect the rotors honestly, replace hardware, and bed in the new pads. If the rotors pass, pads-only is fine; if not, do them together. Start with quality pads and fresh hardware.