The primary benefit of bespoke over off-the-rack is fit. A bespoke corset is made from an individual pattern drafted from your measurements for you and you only. At least one mock-up will be fitted on your body and every curve will be checked. For people with pronounced curves, extreme hip to waist differentials, long or short torsos and so on, off the rack (or even corsets made to a couple of basic measurements) are unlikely to fit. A badly fitting corset is a best unflattering and at worst extremely uncomfortable and even potentially harmful. Anyone undertaking waist training for body modification is also strongly advised to opt for a fully customised fit.
The other reason to go bespoke is the opportunity to be completely and utterly free with the design. A chance to work alongside an expert to achieve just what you want, unfettered by limitations is a fabulous thing. Books of fabric swatches are on hand, and Alison has a few more unusual sources up her sleeve for distinctive fabrics and trims. Vintage and antique pieces are an option as are fabrics sourced locally. Then there are options for embroidery, beading and so on. All resulting in a truly individual garment.