I agee it depends upon the degree of visual impairmen. If you have little sight, the concern without the announcement is, if you are the only one getting off from a given set of doors, being able to locate the 'open door' button in time before the train departs. If you're not even sure which side disembarkation is taking place, that's an extra delay.
This is the sort of issue where the awareness of on-train staff can make a big difference.
If a ‘guard’ or ticket examiner is passing through the train at regular intervals and/or has been alerted at the boarding station they can be well positioned to assist the visually impaired passenger to alight. Besides left and right this can include advice about gaps, low platforms, slippery surfaces and so on.
I’ve witnessed a situation where a DOO Southeastern service arrived at Lewisham (a particularly awkward station) and the driver was clearly aware of a VIP who would need to alight. After a short while at the platform he made an announcement to the whole train explaining the situation and invited all passengers to look out for anyone near them who might be having difficulty in locating the doors or alighting.
On a loosely related note; are assistance dogs trained to be able to identify illuminated door buttons or open doors? I’d not thought of that before.