Used to be that special libraries were the ones always asked to show their return on investment and justify budget expenditures - it was obvious that universities needed libraries and they were judged on the number of volumes (like that was ever a sensible metric for the value of a school!). In the past decade or so public libraries have been under more pressure to show ROI and they do so by showing economic benefits to the community from having library services in an area (there are also many more dimensions used in a nuanced way - see iPac's work). There's a nice (if a tad dated) review by Cory Lown and Hilary Davis here: http://www.inthelibrarywiththeleadpipe.org/2009/are-you-worth-it-what-return-on-investment/.
The term SDI - selective dissemination of information - was coined in the 60's but no doubt librarians have always performed this function. Whether formally or informally, we are asked to keep a look out for things of interest to our customers/users/patrons, etc., and bring them to their attention. Formally, we might have a budget we charge our time or even resources to and we do a detailed reference interview in which we establish the topic but also the type of information desired, the treatment, time period, frequency, and some gauge of whether the person would rather have some junk but be less likely to miss something (high recall) or is ok with being more likely to miss something but wants only things that are likely to be very relevant (high precision).
With this information the librarian might just set up a search in a database, tweak it a bit, and then walk away. She might have the results come to her and weed them every week. Alternatively, she might actually schedule some time to read the news and look out for things and then write up summaries. Informally, it might be just that a librarian knows a customer's interests and if/when she sees something, she forwards it.
Once a database alert has been set up and is running, further intervention is only needed if the database vendor changes or if there's a request. The problem with this is that the end customer can (and often will) forget how they came to get this useful little email every week. We found when we needed to clean up our Dialog account that there were alerts from the former librarian who died in maybe 2002 or so (before I got here). They were super useful to the users and they passed them around within their group, but we were able to re-write them using our current site license to the database and save that money. If there wasn't a bill, we wouldn't have known and certainly those engineers had forgotten.
So what if one of those alerts had a gem in it that the recipient wouldn't have heard about otherwise and that caused them to start a new research program or innovate on a project or save money or .... ? Would the library, or more importantly, the people who pay for the library ever hear about it? No.
For the informal mode in which we keep an eye out for customers. That can be really hit or miss. Sometimes there's all kinds of interesting things going on and other times there's nothing. Maybe we point out 100 things of interest for 1 home run. Maybe allowing ourselves the time to look - to read the news, the industry newsletters, the science magazines (like society member magazines like Physics Today, EOS, etc) isn't do-able. That's a huge problem. It looks like you're doing nothing but fooling around on the internet. When you do send something good, they might be like "great - send one this good every week!" or "more like this!"
We were going to start up sector newsletters here, but it's really not sustainable because you have to look around and read for a while to see new and interesting things worth alerting people on. Sure, it's super useful but how many hours go into each home run? The bosses very much appreciate these tips they get, but they do not want to pay for the time for people to look for the information.
My old boss used to say that we needed to be just-in-time not just-in-case and that's total baloney. Libraries by definition are just-in-case. These alerting services are just-in-case. Metrics like number of alerts sent out are not useful. Stories of times these alerts were truly useful and used are great - but you have to hear them and record them.
My library has lost some big battles in justifying our existence so I am clearly not that effective at this. It's a sticky question, I think. My blog posts always peter off like Crichton novels, but oh well. Happy New Year - hopefully we'll still be providing library services in the new year after we're re-organized again, sigh.