The harness will keep you safe in an accident and will reduce your fatigue if it's a proper snug fit.
I strongly recommend Enduro style belts as they are far easier for the driver to adjust solo.
Originally that meant shelling out $600+ for Scroth Enduro Belts. Fortunately other manufacturers offer more affordable alternatives: RaceQuip FIA Camlock Harness.
The other key decision you nede to make is whether to get FIA belts thta last five years and then have tio be discarded or non-FIA that need recertified every two years. I use FIAS belts but I have friends that swear by Safecraft and Crow because they will recertify their belts (for a fee) and have excellent customer service.
From the rules:
"3.6.2.1 Five- or Six-Point Harnesses Mandatory. Five- or six-point harnesses are mandatory, including a fifth or fifth/sixth “anti-submarine” belt. All harnesses must be in excellent, near-new condition, properly mounted, and carry SFI or FIA approval tags. Harnesses with expiration dates are not valid after the expiration date. Harnesses with a manufacture date but no expiration date are acceptable for two years after manufacture. Shoulder harnesses must be two totally separate belts with separate mounting points (i.e., single-point Y-belts are not allowed). When viewed from above, shoulder harnesses should be closer at their mounting points than at their seat-entry points. All lap belts must be standard 2-inch or 3-inch width.
3.6.2.2 Harness Mounting Hardware. SAE Grade 8 or Metric Nut Class 8.8 or better hardware and 2.5-inch or larger load washers are required when mounting to sheet metal.
3.6.2.3 Anti-Submarine Belt Mounting. Anti-submarine belt(s) should be mounted in accordance with manufacturer’s instructions.
3.6.2.4 Harness Routing. Belts should be routed and threaded as shown in Lemons’ “HOW TO NOT FAIL LEMONS TECH INSPECTION” PDF, with at least a 4-inch tail. All sliders should be snugged up to their mounting plates or harness bars as much as possible. Belts should be neatly and evenly folded when passing through narrower hardware, such as 3-inch belts passing through 2-inch mounting plates.
3.6.2.5 Snap-Type Harness Ends. On snap-end-type belt mounts, restrain the snap arm with a cotter pin or safety wire through the hole in the arm."