It matters because it's the difference between dealing with the situation at hand, or authorially changing the situation to something else.
The situation is "alley with wall to climb". How do we know whether or not their are boxes present?
If you assume that the gameworld contains nothing but what the GM has already described, orally to the players or in prior writing in his/her notes, you will get a gameworld that is absurdly austere compared to the real world. Walk around an inner urban neighbourhood and look at all the stuff in the streets, in the lanes, on the verandahs, visible through the windows, etc. Or, if you think those results will be exaggerated because of the increase in material production between mediaeval and present-day times, look at a list of household contents in a decent history book.
Now look at the descriptions provided in the typical module or world-setting. Either people in D&D don't comb their hair, sew their clothes, feed their horses, butcher their meat, etc, or alternatively the gameworld contains a whole lot of combs, needles, hay, knives, etc that are not being described in those notes.
At which point, a decision needs to be made when the players ask - for instance, if it's not "Are their boxes in the alley" it might be "Is their a horseshoe nail that I can try and use as an improvised lock pick?" or "Is their a skillet that I can bang loudly to attract attention?" or "Does the temple have a scholar whose fluent in ancient languages?" or any of the other hunreds of ideas that players might come up with to take advantage of the bog-standard stuff that is found in streets and houses and cities.
In these circumstances the GM
has to add new backstory elements. And I'm still asking you to cite some GMing advice that says that, in doing so, the GM should not have regard to what will be interesting to the players.
"The referee should set the throw required [on 2d6] to obtain any item specified by the players" is no different than what folks have been saying all along regarding a player asking about something and the GM randomizing the possibility if it isn't already delineated.
It's quite different. First, the
player gets to specify the relevant ingaeme element. Second, the PC's Streetwise skill is a DM on the roll, which is made by the
player. The GM sets a DC, but does not get to randomise the outcome.
In other words, it's the player using a PC skill (Streetwise) to introduce content into the shared fiction, although the PC is not him-/herself the ingame creator of that content.
There is no functional difference between this and the BW Circles mechanic. And the only functional difference between this and an OGL Conan-stye fate point system is that the latter allows auto-success whereas the Traveller mechanic requires a successful skill roll.