I have quite a lot of photos from 1890 - 1930 of my ancestors. They are mainly family snaps but some have historical buildings in the shot. All are from Australia. Is there a benefit to donate them to National Archives or similar?
Donating your photos to a reputable archival institution is a great idea if:
Historical societies and general members of the public will especially enjoy having access to images of local buildings. If you are concerned about the images being preserved, then this is a great idea and you probably will receive a tax receipt once the donation has been appraised.
Well, the society as a whole will probably benefit from your donation.
When those photos are hold just in your family, or just with you, people who visit the "National Archives" of Australia (or any other institution) don't have the oportunity to see how a family looked like about 100 years ago, or how some building was at that time, and so on.
Another point is that those photos will be preserved by specialists, and perhaps you don't have all the resources and knowledge to preserve those pictures at your home in the same conditions.
If you don't want to "lose" those photos to a museum, etc., there are alternatives. Take a look at the Temporary Loans of Original Materials and Our Interest in Copying Family Collections, for example, from the http://www.ushmm.org/research/collections/photo/ . Despite their interested in a limited set of photos, the idea remains: you can contribute sharing those documents and still have the originals in your family. I'm almost sure that there will be institutions in Australia interested in those alternatives.