Thinking about this a little more.
It probably was not intended as anything more than a temporary wartime occupation to compel Spain to sue for peace, with the US unlikely to insist on annexation.
But let's play along, and say that the United States demands and receives the conquered and occupied islands in the peace treaty, and they are administered as an insular territory, possibly under some sort of regime similar to that imposed on Puerto Rico.
The United States now has a trans-Atlantic possession offering naval bases to project US power to the eastern Atlantic and not far from the Mediterranean.
It can protect the vital American trade road to...pretty much nowhere...the people-less and penniless Sahara desert and Morocco.
Now that's a bit of an overstatement, with sovereign coaling station and repair base, the US Navy's effective patrolling radius on a short term basis in the neighborhood of Western Europe, Western Africa and the Western Mediterranean will be improved.
The US will be interested in Moroccan affairs (more so than it actually was). And if the Perdicaris affair is not butterflied away (and the McKinley assassination and TR Presidency are not butterflied away) President Roosevelt can intervene in Tangier in even greater force than OTL
Perdicaris_affair.
Like OTL, I would expect the US to be invited to the 1906 Algeciras Conference on Morocco, and vote with the overall navally and regionally stronger powers, Britain, France, and Spain - over Germany, as in OTL.
Supposing the Great War starts like the historic Great War, US naval forces will more often be close to the early naval action, and perhaps feel somewhat more empowered to challenge the British tightening of the paper blockade of Germany, and increasingly, the neutrals, between 1914 and 1916.
However, geographically, again, the Canaries are nowhere near the North Sea and direct trade routes to Germany, nor the Netherlands, nor the Nordic neutrals. Nor are they near German colonies, or at least ones that don't fall fast (like Togo) or fall under close, effective Allied blockade (Kamerun) rather quickly.
Perhaps direct radio communications between the US and Germany last longer, I'm not sure.
In any case, the US position in the Canaries may marginally improve the American ability to assert its neutral trading rights with the Iberian neutrals, and Italy and Greece - along with some transshipping trade to the CPs, at least as long as some of those remain neutral. Of course, Italy is destined to join the Allies in 1915, Portugal in 1916, and Greece in 1917, pretty much cutting off the US Canaries and Spain from effective trade routes with Germany and Austria and Turkey and Bulgaria.