The Knowledge Transmission Mechanism (KTM) is how knowledge produced by academics and other researchers is translated into public policy. Evidence based policy is the result of this mechanism working. The media is, in theory, an important conduit for the KTM: media publicises research, policy maker sees/hears/reads media and gets their civil servants to investigate research. Or media communicates policy consensus on issue, and politician is questioned by the media on why they are not following this consensus.
The rigid application of political balance in the broadcast media is in danger of negating the KTM, and therefore evidence based policy. The moment an issue (call it issue X) is deemed ‘political’ by the media, balance dictates that any view expressed on issue X is an opinion rather than knowledge. As a result, when the media want to talk to non-politicians (‘experts’) about issue X, the imperative of balance remains.
Now suppose that in the knowledge world there is in fact a consensus on issue X. That would be a problem for balance broadcasting, because it would be difficult to get an expert to argue against the consensus. The BBC overcame this problem valiantly during Brexit, using Patrick Minford (who is not known as a trade economist) time and again to balance the IMF, the OECD, more than 90% of academic opinion etc. But another way of solving this problem is to use certain think tanks.
There are two types of think tank. The good kind can be a vital part of the KTM. There is often a genuine need for think tanks to help translate academic research into policy. Sometimes these think tanks will be very like universities (like the IFS for example). Other times they will be think tanks that have a broad left or right orientation. These think tanks are an important part of the KTM, because they can establish what the academic consensus is, translate academic ideas into practical policy, and match policy problems to evidence based solutions. The IPPR is an obvious example of this type of think tank. They are part of evidence based policy making.
The bad kind are rather different. These produce ‘research’ that conforms to a particular line or ideology, rather than conforming to evidence or existing academic knowledge. Sometimes these think tanks can even become policy entrepreneurs, selling policies to politicians. This is often called policy based evidence making. It would be nice to be able to distinguish between good and bad think tanks in an easy way. The good type seeks to foster the KTM, and ensure policy is evidence based, and the bad type seek to negate the KTM by producing evidence or policies that fit preconceived ideas or the policymaker’s ideology.
I would argue that transparency about funding sources provides a strong indicator of which type a think tank is. Why is this? One obvious reason why you would not want, say, company Y listed as a funder is if you subsequently produced a report that was directly in the interests of Y. A clear example is the IEA’s arguments against plain packaging of cigarettes, and its funding from tobacco companies. To be clear I am not suggesting that the IEA (who have been in the news recently) is insincere about the arguments it makes, but if funding was transparent it would be very easy to suggest their arguments on plain packaging were contrived by just pointing out its funding sources. So that is an obvious reason to keep funding secret. In contrast, an IFS type think tank has no reason to hide its funders, because it is not in the business of producing policies that meet the specific wishes of these funders. 
It has been suggested to me that IEA type think tanks need to keep their funding secret because some personal donations would get individuals into trouble with their employers if they became public. It would be interesting to know from some IFS type think tanks how much funding they lose for this reason. Perhaps the answer is not much, and that in any case the principle of transparency is more important than a few extra donations.
Another good indicator of a bad think tank is their relationship to academia. I have told the story about how the IEA under Philip Booth tried to cultivate the idea that the 364 academics who famously objected to the Thatcher government's 1981 budget were embarrassingly wrong, when in fact they were proved right. The IEA’s media and political connections are sufficiently strong that even good BBC economics journalists were taken in by their line. To the extent that it emboldened Osborne to ignore the majority of academic economists over austerity it was a dangerous myth to cultivate.
In the case of global warming the BBC has been forced (I don’t think that is an inappropriate word, as it often breaches the guidance) to treat man made climate change as a fact rather than an opinion that always has to be balanced. That is not going to happen for some time over any economic issue, however strong the academic consensus (like Brexit). This is partly because the pressure from academia is much less, and partly there is still a prejudice against social science (as if evidence based policy making cannot occur for economic or social policy!). But the BBC does need to explain their attitude to the use of think tanks. Why do they use think tanks that do not declare their funding sources, and when they do why is this information not passed on to its audience?
 To understand why arguments like the IFS gets money from the government or the EU and therefore it is biased do not work, see here.