Firstly, not all sites disallow recommendations. Mathematics, TeX, Mathematica, and Reverse Engineering are some sites that I know of that allow a controlled form of recommendations (I think). Physics is also on its way to allow a subset of resource recommendation questions.
That aside, each site has a different type of recommendation question that is common to them. Stack Overflow has library and book recommendations. Security.SE has broad "strategy" recommendations, along with best practices questions and some product recommendations. Super User has product recommendations. Physics has book/website recommendations. Chemistry has book and software recommendations. The old Astronomy.SE used to have physical tool recommendations (telescopes, lenses, etc). Scifi has "what should I watch next". Bitcoin has software recommendations. Electronics has hardware recommendations and where-should-I-buy-this questions. English has word recommendations.
While each of these has the same underlying theme to them, they're different. When the close system was put in, most, if not all, sites were given the following close reason:
Questions seeking product recommendations are off-topic as they become obsolete quickly. Instead, describe your situation and the specific problem you're trying to solve.
But this one is pretty short and not ideal (in fact, it doesn't even make sense on some sites where the recommendations are not "product" recommendations). Sites should identify what type of recommendation is common for them, and tailor the close reason to the type of question they get.
For example, Stack Overflow has:
Questions asking us to recommend or find a tool, library or favorite off-site resource are off-topic for Stack Overflow as they tend to attract opinionated answers and spam. Instead, describe the problem and what has been done so far to solve it.
Super User has:
Questions seeking product, service, or learning material recommendations are off-topic because they tend to become obsolete quickly. Instead, describe your situation and the specific problem you're trying to solve. Here are a few suggestions on how to properly ask this type of question.
Questions seeking recommendations for specific products or places to purchase them are off-topic as they are rarely useful to others and quickly obsolete. Instead, describe your situation and the specific problem you're trying to solve.
Questions on choosing an ideal word or phrase must include information on how it will be used in order to be answered. For help writing a good word or phrase request, see: About single word requests
Requests for lists of works or recommendations are off-topic as they do not fit our questions and answers format. Feel free to ask about people's favorites in chat.
One can see that they all vary, and these specific reasons are much better than the original catch-all.
Some sites don't have that large a problem with recommendation questions, so they don't even have a close reason for it. Instead, they close them as "primarily opinion based" or they use the freeform off topic close reason when these questions come in. Physics has bigger problems with homework and non mainstream posts, so we don't have a close reason for recommendations. Skeptics has a whitelist of the types of allowed questions, instead of using the usual blacklisting system -- only questions asking about the veracity of a notable claim are allowed, and that pretty much excludes the other types of bad questions like lists (they only have one close reason, and that's all they really need). Christianity doesn't seem to get many recommendation-type questions. There probably are more like this.
So the importance of the recommendation close varies from site to site, as well as the type of question it actually targets.
What sites are encouraged to do is to take a look at the recently closed questions, and determine the types of commonly closed questions. The community team has some queries that can help greatly with this. First, identify is recommendation questions are really a problem for you. If that is the case, then identify the type of recommendation question you get, and propose a custom off topic close reason for it. The number of slots can be increased from 3 if you can demonstrate a need.