Changing terms and language is an important part of destigmatisation campaigns. Whether you refer to Downs Syndrome or "chromosomal difference" makes no material difference to the actual condition as it is experienced, but 'chromosomal difference' doesn't carry anything like the number of associations with stigma, prejudice, ignorance, deliberately cruel language, or misguided and preconceived notions that 'Downs Syndrome' does, so changing terms can be an important part of dispensing with or minimising societal stigma, and can therefore lead to a small but significant improvement in the lives of people living with the condition or the relatives and carers thereof.
Society doesn't commonly describe people as 'Schizophrenics', 'lunatics', 'hysterics' and such any more because those terms are outdated, carry a stigma, and centre the condition rather than the individual. It's going the same way with 'wheelchair user' and such, where the emphasis is again on the wheelchair and not the person. It's slightly different with terms that describe a condition or diagnosis, but the likes of 'Downs' has long been used in a pejorative way, so I can totally understand why people might believe there's a need to move on from it. We don't diagnose people as 'morons', 'cretins', or 'idiots' any longer, but those terms also have their origin in the medical model, albeit an archaic and flawed one.