Brake cleaner is one of those cheap shop supplies that is easy to skip - but doing a brake job without it usually means noise and contamination problems. It is a fast-evaporating solvent spray made for exactly this job. Here is what it does and why you want it.
What brake cleaner does
Brake cleaner blasts away grease, oil, brake dust, and old anti-seize from brake parts and evaporates fast leaving no residue. The two big uses: cleaning the protective oil off new rotors before install - see clean new rotors before install - and cleaning the caliper bracket and hardware seats so new clips fit and pads slide freely.
Why it matters for a quiet brake job
Any oil or grease left on the rotor face or pad causes uneven grip, squeal, and contamination that ruins the pad. A quick spray-down of the rotor and a wipe removes the factory coating and handling oils so the pads bed cleanly - a big reason cleaner-skipped brake jobs end up noisy; see brake noise after new pads.
How to use it safely
Spray the rotor faces and wipe with a clean cloth until it comes away clean; spray the bracket and slide-pin bores. Keep it OFF the pad friction material and off rubber boots where possible. Work in ventilation - the fumes are strong - and keep it away from flames. Let parts flash dry before assembly.
Do not confuse it with grease
Brake cleaner removes lubricant; you still need the right high-temp brake grease on the slide pins and contact points after cleaning. Clean, then grease the correct spots, then assemble with quality pads and fresh hardware. It is a couple of dollars that prevents most brake-job complaints.