First off, I think we can all agree that the Internet is an amazing thing. It is immense and full of things you would never think of if you did not see it. The Internet contains some amazing pieces of history for the world to enjoy. Some, like the Virtual Hampson Museum, have 3D scans of archaeological artifacts for perusal by anyone who wants to look. Others like the podcast series Stuff You Missed in History Class make a concerted effort to spread accurate history from around the world that is not always a part of high school curriculum.
The Internet has become not just a place for entertainment, but also a place for scholars to gather information and educate the public. Now archivists and museum workers can scan objects in order to create a digital record. That record can be corrupted or cease to function, but it can also preserve an object on a platform that makes it more accessible than it otherwise would be. Digital records and scans can work as backups in case the original get destroyed. True, it would not be the same as having the original, but a copy (especially a high quality copy) is better than nothing.
I believe that one of the big problems with keeping copies or scan of historical artifacts online is the same as with most things online: someone taking something out of context or lying. If a scan of a historical picture is online then people are able to copy and paste or take a screenshot. Normally that is not much of a problem (easy access is one of the good things about the Internet, after all.) but it can be an issue if the person using that copy then reposts it somewhere with no context or with fake context. But again, this is an issue that most of the Internet has to deal with.