Buying an off-the-rack suit can limit your options. They're all in generic sizes, so the odds are you'll end up trying on several and shopping around at different stores. And at the end of the day, a salesman will probably talk you into a suit that seems just acceptable to you - not great - just acceptable.
Furthermore, you're forced to settle for a pattern and a cut that may not be very flattering for your build. Not to mention the definite difference in the level of quality. Think about Hundreds of fabric options instead of a few. Most off-the-rack suits are put together by machines and glue on an assembly line.
Whereas Brooks & Sons pride themselves, because every custom-made suit is put together by a tailor who's attention to detail cannot be matched. If you still think an off the rack suit is "just fine" remember, "just fine" to Brooks & Sons is a failure. Tailored suits not only feel like a second skin, but they also accentuate your best features while masking those you'd rather keep hidden.
Ask any man who's taken that step and you'll quickly realize that once you visit Brooks & Sons, you will never go back to the rack.