Almost everyone who loves knowledge, history and understanding the birth of humanity's great cultures will be horrified by pictures of IS reducing ancient statues to powder in occupied areas of Iraq. What historical knowledge is being destroyed along with these artefacts, aside from the very obvious loss to humanity of the sheer visual impact of them, and what kinds of pre-Islamic civilisations do/did they represent?
Finally, is there anything to be said for why they are destroying symbols of these specific civilisations (with reference to, say, evidence about the origins of Islamic belief that they may find unpalatable), or is this just about destruction of "idols" of any belief system?
While the main part of this question is fine - "What is the value of the stuff being destroyed?", I would just like to remind any potential commentators that, while this is a tragedy, this is not the correct platform for discussion of ISIS activities, which run afoul of the 20 year rule
ISIS follows a particularly form of Salafi logic about what counts as shirk, a term often translated as "idolatry", but more literally carries the sense of "giving God an associate" or "attributing a partner to God". Some streams within Salafist thought have taken a particularly hardline on shirk, and have associated things like Sufi tombs as "shirk". This stream has, for them most part, only become dominant anywhere in the very recent past (i.e. in the last twenty years, off limits for further discussion, but suffice to say you saw Salafi-influenced destruction in Mali and Afghanistan). It has been going on slightly longer in Saudi Arabia, where Salafist thought has played a much more prominent role historical than other places (Salafist thought has long been associated with the Hanbali school of law, but it is not exclusive to that school). Arguably, the House of Saud, in alliance with hardline Salafist clerics, have allowed or encouraged the destruction of historical sites (mainly Islamic ones, interestingly) since the beginning of the 19th century, this once again picked up when the Third Saudi State (the current state of Saudi Arabia) was formed, and the House of Saud was once again in power. Even then, though, my sense is that this shirk-influenced destruction of historical sites has only really picked up since the 1970's or so, and has only really taken off in more recent years. The best easy source on this is probably the Wikipedia page on "Destruction of early Islamic heritage sites in Saudi Arabia". It's complicated, though, because unlike the recent non-state groups that have taken to destroying historical sites who clearly define their actions in terms of shirk, the Saudis often make arguments based on the huge increase in the number of people making Hajj over the last few decades (because of economic development, internal combustion engines, highways, and above all jets) and the consequent need to modernize the infrastructure of Mecca and Medina to allow for the increase in pilgrims, without explicitly reference to specific Salafist theology (commentators pretty much universally read between the lines of these policies; I don't have any handy references that cover the issue from more than 20 years ago, unfortunately). So, in short, Salafist ideology doesn't call people to destroy any civilization's artifacts specifically (indeed, Islamic art and history is in places the first to go), but anything that is deemed shirk or possibly shirk. Most other streams of Muslim thought (and, indeed, likely even some streams of Salafist thought) do not hold by this broad interpretation of what constitutes shirk.